Historically, for Reformed Christians, theology has been seen as the foundation to philosophy, as well as other disciplines. The reason for this is that theology, with its content taken from God's revelation, is able to provide the foundations that other disciplines cannot provide. God's revelation tells us that He is the Creator and Sustainer of all that is. He alone existed "in the beginning" and thus His character transcends everything else that exists. Not only so, but He is the one who gives the facts of the world their meaning. Without Him, there are not facts; without Him, there is no universe! He is the principium essendi — the source of existence — for everything else that exists. In metaphysics, therefore, philosophy should begin with the distinction, and relation, between the Creator and creation.
In epistemology, philosophy should begin with the fact that this God who created all that is, also spoke in and through His creation. Since all knowledge has its genesis in God Himself, human knowledge is justified to the extent that it is consistent with what God has said, in His Word and in His world.
The "big questions" of philosophy — what is the nature of reality (metaphysics)?, how can we know anything (epistemology), what is the nature of right and wrong (ethics) — can only be properly discussed within a context that acknowledges the necessity of God's existence and His revelation to creatures made in His image. Because the universal problem of sin has erected a barrier between God and humanity, a proper discussion of philosophical topics requires the regeneration of human hearts, which is only accomplished through the gospel and its application by the Holy Spirit.
K. Scott Oliphint, Ph.D. is professor of Apologetics and Systematic Theology at Westminster Theological Seminary.
For Professor Oliphint's full treatment of this question:
K. Scott Oliphint, Christianity and the Role of Philosophy.
Christianity and Philosophy: Four Views, ed. Gould and Davis.
Professor Oliphint's recommended extra resource on this question:
Debating Christian Religious Epistemology, ed. DePoe and McNabb.