Having now seen many successful--and more unsuccessful--PhD applications from gifted students, I shall attempt to offer guidance for those interested in a PhD in theology, especially at Wheaton. Some, or even much, of this advice is transferable to other contexts. And there are others who have written useful counsel that is applicable for discerning your particular ministry calling and applying for doctoral work in a range of fields.
See, e.g., http://seanmichaellucas.blogspot.com/2008/05/ministerial-students-calling-and-phd.html and http://nijaygupta.wordpress.com/phd-advice/.
Nevertheless, the first lesson of PhD preparation might well be to study very carefully the particular dynamics of the program(s) in which you have interest. And the second lesson might be to realize that a PhD is very hard, in many ways. That's partly due to academic dysfunction. But this is a reality even at Christian schools. And PhD programs are hard because academic life is hard, so you should only receive the credentials if you can handle the reality. Being a teacher-scholar is full of profound delights but also heavy demands. Students often glimpse the delights while neglecting the demands. Joy comes when you find that God has called and gifted you for both.
Discerning Whether to Apply
A PhD requires several years of time, which requires money and/or debt. Make sure your family is supportive and that it has the stability and infrastructure to handle these realities. (a) Do not be naive about how much money you'll make once you have the degree. You'll probably start out making $50,000 per year if you're lucky, and depending on the cost of living where you land, that doesn't leave a lot for paying off loans. (b) Do not be naive about how much money you'll have during the degree. Relatively few schools provide stipends for students in biblical and theological studies; by no means do all schools even provide free tuition. Often these schools lie in metropolitan areas where cost of living is not cheap. Students who want to "start a family" during their degree face additional pressures of various kinds, not just financial. It is naive to think you'll live comfortably in a nice-sized house during or even after the degree.
A PhD requires other forms of perseverance, including the right blend of humility and confidence to enable you to weather the storms of competition and struggle with a quantity and quality of work you've never faced before. Then, after the degree is in sight, you have to face the job market!
A PhD requires high academic aptitude, not just interest in studying more or the ability to get decent grades from your master's level professors. Grade inflation is a common reality that can mislead you. Moreover, most master's level assignments--especially in seminaries--are structured very differently from doctoral level work. Thus many well-meaning applicants have no conception of what a PhD program means by "an original contribution to scholarship" (see below).
To be blunt, then, if you are considering doctoral work in the U.S. you should take the GRE. If your scores are 700+/700+/5.5+, then you might be competitive for a top-tier program (e.g., Notre Dame; Princeton Seminary; Duke). If your scores are 650+/600+/5+, then you might be competitive for a second-tier program (e.g., Marquette; Loyola; Wheaton; TEDS). If your scores are significantly below 600/600/4.5, then you should not bother applying. If they are in a gray area between the numbers I've mentioned, then you might be competitive for some second-tier programs depending on the rest of the applicant class in that given year. I say "might" for all of these categories because test scores are only a threshold through which one passes to start a more competitive process; they are not a guarantee of admission unless they are exceedingly high, and then only at certain places.
Moreover, you should "count the cost" in general. I often tell students, "If you can imagine yourself doing anything else besides a PhD, then do it." The job market suggests that in most fields we evangelicals do not need more applicants; we need a few better-prepared ones. In the church, meanwhile, we quite likely need more intelligent and intellectually-curious pastoral staff members. Let the one who has ears, hear.
Having said that, I had a seminary professor tell me that I was not well cut out for pastoral ministry in certain respects, whereas "if you don't go into academia, you're wasting your gifts." If someone tells you that, then again let the one who has ears, hear.
Perhaps the bottom line, then, is to ask your potential recommenders to be really honest with you before they simply agree to write reference letters. You will need to give them the freedom to do this, because--speaking from experience--it is not easy to tell someone whose heart is set on a PhD and teaching career that you don't think they're cut out for it. But you need someone to care for you enough to be as honest and helpful as they can.
Determining When/Where to Apply
You need options, especially if you are applying to top-tier programs, which frequently face a range of considerations beyond test scores and references. Diversity concerns, balanced supervisor loads, upcoming sabbaticals, and so forth can affect admissions. So don't apply at just one or two places, unless you have your heart set on something in particular and you're willing to wait.
While a school's reputation is important, I believe the top consideration in your decision should be your potential supervisor(s). You are going to apprentice yourself to this person for years. At minimum they need to be willing to supervise someone with your doctrinal commitments, piety, and interests. At maximum they should be people you would be delighted to imitate in teaching and scholarship ... and life. Ideally the person would have a reputation that would make their reference letters useful in the network within which you want a job. Moreover, you should visit and/or ask current students how approachable and available they are. Is this a person who would make you wait six months for feedback on a dissertation chapter? who would refuse to read your work or meet with you during a sabbatical? Etc. These are real-world scenarios; I'm not making them up.
If you have matched a set of schools to your interests and aptitude (test scores and so forth), it is worth contacting one or more potential supervisors at each. They can steer you in helpful directions for the application process. You might also learn how approachable and available they are.
There is no magic formula for knowing when to apply. Some students need a year or two off, to recharge batteries and avoid burnout, or to experience vocational ministry, etc. Other students know where they're going, find school energizing, and get enough breaks already, perhaps deciding not to pursue ordination, etc. They should proceed full steam ahead.
Do not be naive about how much study and further preparation you'll achieve in a year or two off. The accountability structures of school are often there for a reason. Relatively rare is the student who uses time off well, academically speaking. Yet some do, and sometimes you're not ready without this time--to improve or gain languages or test scores, for example.
Do not be naive enough to believe that you'll ever feel academically ready. Unless you are battling pride, you probably won't. At a certain point you have to trust others that you have the basics of an application in place, and it's time to go for it. You can always look around you and find someone else smarter or better prepared; don't give in to this form of acedia.
Developing an Application
Much of what I need to say here has already emerged above. Get good GRE scores and references--people who know you well, with at least one in your particular field, if at all possible. Get as many languages in place as strongly as possible. Build some preliminary contact(s) at the various schools, and learn the nuances of what particular programs ask for in their applications. PRAY! But still there are a couple more crucial elements ...
Writing sample. Pay attention to guidelines a school provides, for instance regarding length. Not all schools will throw out samples that are too long, but some probably do and others are tempted! Professors on admissions committees have enough grading to do already; they aren't interested in reading tons of papers. They will skim the samples of students whose test scores and so forth interest them. They will be looking at the intros and conclusions to see if you can set forth a clear thesis and plausible, cogent arguments. They will be looking at the middle to see if you cite a range of good sources and if you cite enough to be scholarly but not so much that you parrot others and demonstrate no creative thought of your own. It goes without saying that if you provide a document which adheres to no style/format guide, while being replete with spelling and grammar errors, you should not expect your application to go further. It is ideal if your writing sample bears some significant relation to the proposed subject of your doctoral research. Many are the theology applicants who send to Wheaton writing samples of biblical exegesis. To be sure, this is an essential theological skill. But it is not the whole of the enterprise, and it is difficult on that basis alone to discern whether a student has aptitude for the task of historical or especially systematic theology.
Application essays. It is possible to cut and paste considerable amounts of text between various applications, but again do not neglect to take into account the nuances of particular programs. Application essays are your chance to "spin" yourself--honestly. This means that if you have one or more perceived deficiencies, these essays offer a chance to interpret them for the committee. For instance, if you got a C in first-semester Greek, was this due to bad language aptitude? or instead to family trauma, acclimation to seminary, too many credit hours, etc.?
Dissertation subject. For certain programs--especially in the U.K. and at Wheaton, with its attempt at a hybrid U.S./U.K. model--your proposal regarding a research agenda is very important. The general principle is that you don't want to have a dissertation proposal entirely worked out, because the school/supervisor expects to have some input within the first year (at least). On the other hand, you don't want to convey that you have no idea what you're doing. Some schools/supervisors use this as a weeding-out mechanism; I had one famous British New Testament scholar tell me that whether or not students could find a thesis topic that needed doing was a crucial way in which he found whether or not a student deserved admission. Other schools/supervisors are willing--indeed, prefer--to have the process be dialogical, perhaps in the year of application. The student states a fairly broad area of interest and the professor gives a hint or two about how to focus it; then the student writes again with a more focused version, with the professor giving a "right-track" or "wrong-track" response before the student writes the application, etc. Wheaton probably fits this latter model more than the former, though I admit I have received some emails from students which indicated they were so unfamiliar with the concept of writing a dissertation that I basically crossed them off my mental list of prospects. How then to proceed?
Many students become interested in biblical theology and/or narrative approaches. As a result they propose to throw out more traditional systematic theology or "dogmatic" approaches and to develop a "whole-Bible" redemptive-historical synthesis on a subject. This is almost never possible within the narrow scope of a dissertation. It also usually reflects a lack of understanding regarding systematic theology as an academic discipline, and what counts as a contribution to scholarship therein. Such students need to recognize the acceptable scope of a reading area for beginning dissertation work. Such a reading area is not simply a subject of interest--say, the arts--but includes a more particular manner of approaching the subject: say, Wolterstorff's theory of art; the arts and evangelism; the Bible and music; the arts as rhetoric; etc.
Other students send in writing samples and/or proposals that contain long bibliographies indicating they read extensively within such an area. But these students provide only description--a survey of literary territory. Often they think that such a description--of, say, a biblical narrative approach to the arts--would be tremendously helpful to the church. And it might, but it may already have been done in the academy. Sometimes master's theses don't go much farther than this either; their argument consists in providing the state of scholarship on a given question. A dissertation, however, must press on to a research question that either no one has asked before, or else no one has answered satisfactorily, or about which people currently disagree, or about which people have not talked in a while.
You cannot read everything relevant to an area before framing a research question, let alone writing a dissertation. What may help you to move from a reading area to a research question is to understand the disciplinary structure of academic organization. Reading areas may overlap several academic disciplines or subdisciplines. An original contribution to scholarship answers a research question about a reading area from a particular methodological perspective and for a particular disciplinary audience, even if others will be interested. Thus, if you propose to describe and analyze Wolterstorff's theory of art in some new fashion, your project is a "historical" one; if you propose to build on and apply Wolterstorff's theory of art in some new fashion, your project is a more "systematic" or "philosophical" one. Of course these are fuzzy boundaries, but boundaries they are--at least in the academy.
Accordingly, your thesis proposal needs to be clear about the primary method(s) by which you can answer the research question in a distinctively new way. At a broad level, to describe/analyze Wolterstorff's theory of art is of interest in the field of "historical theology." However, it might also be of interest within a larger project of "systematic theology," if for example you need a theory of art as action to form part of your approach to the role of the arts in evangelism. The crucial issue is what methods will be convincing to what audiences at what parts of your thesis.
In systematic theology the descriptive/analytic work that some might call "history of doctrine" can be a major methodological component of a project. But usually you will need to press on at some point, developing implications of the new historical understanding. Let's say you establish what Wolterstorff says: so what? Is he right? And what new insight does he provide relative to others' approaches or in this new area of conversation?
The upshot of all this is that you cannot simply send in a dissertation reading area; yet, probably, you are not able to prepare a fully developed research question. In between you do, however, need to manifest that you have done enough reading in a subject area to know who some major players are and what a major issue is that you want to tackle. A research question will emerge more fully formed once you discern the primary method(s) by which you will make a contribution to the discipline of systematic theology. At the application stage, though, you should at least work to indicate that what you're proposing has not yet been done, and why it is worth refining and doing. So, for instance, you would probably find several analyses of Wolterstorff's theory of art, but few or no instances of its application to the arts in evangelism. Is this worth a dissertation, or only a journal article? Probably the latter, unless you find a way of expanding the material--via other authors, a fuller defense of the theory dealing with Scripture and/or recent responses to Wolterstorff, implications for concrete practices, or perhaps expansion from evangelism to a broader motif such as mission or the church's cultural engagement.
If all this seems very fuzzy (and often it does), then there are some practical ramifications for you. One, go to the library and obtain (perhaps via interlibrary loan) dissertations to examine. You need to see what they are like. Two, read renowned books in your field, examining them especially for their structure and methodological setup. Books that began life as dissertations may be particularly helpful. Three, accept the fact that most of your master's level papers--and maybe even a thesis--did not demand the kind of work a dissertation does. They may have required a heavy amount of work. But writing up descriptive research synthesizing secondary sources or even expounding primary texts is not the same as contributing creatively to a discipline. You need to learn to sit still with scratch paper--or think in the shower--about your own approach to the material before or alongside dealing with what others give you.
Alternatively, the point is not that dissertations leave description or analytical exposition behind. They don't. Dissertations largely consist of such. What makes a dissertation, aside from sheer size and scope, are the argumentative summaries and transitions that stitch the descriptive elements together and, we hope, add up to a fundamental creative insight. This insight, however, may seem to you quite modest relative to the time and paper you devote to the project. Don't be discouraged that dissertations often involve detailed legwork to prove what is fairly predictable or obvious. That legwork itself may be structured in such a way as to constitute a creative contribution, because these contributions are not made simply to a subject area but to the state of scholarship on a given subject area. That is why, if you don't already understand how the theological academy works, to do so is your first step in crafting a dissertation proposal.