Clearly in discussing faith and reason it is imperative to come to adequate definitions. Unfortunately, this will not be possible within the limits of this post. Rather, at most we may point to some problems to be avoided, then look to see if there is any cooperation between faith and reason. If there is, then despite some of the problems noted in previous comments, we may begin to find enough compatibility between philosophy and Christianity that we may begin to affirm, if not to construct, something called Christian philosophy.
It seems inescapable to me, and to the ancient skeptics, that there is no possibility of the exercise of reason without an inherent faith that reason is capable of the apprehension of reality. Whether this is the same sort of faith that Christians exercise in their belief in the existence of their God and Lord, Jesus of Nazareth may be open to some debate and qualification. But the point is simple: reason necessarily acts on the basis of some level of faith in reason to do what reason claims to do. It is, as Sextus Empiricus summarily noted, a rather vicious circle. But there it is: There is no reason apart from some sort of faith. More to the point, there are certain “dogmas” of philosophy which reason cannot properly justify but which may well be nonetheless necessary for reason to do certain of its works. Or at least so thought Immanuel Kant relative to the concepts of the soul, God and freedom of the will. Perhaps Kant himself was mistaken, but in his great architectonic explicating pure reason he recognizes certain paralogisms and antinomies that cannot be gotten round and yet must be in some way granted to continue the rational project.
In at least this regard the ancient and pejorative debate over fideism and rationalism may be put to rest here. We need not tolerate such a dichotomy.
But let us be careful to also acknowledge that the faith reason requires is not necessarily identical to the faith Christ requires. The faith in reason is largely an askesis of intellect; that in Christ is an askesis of love. Yet, as we have said, both are productive of their respective forms of knowledge. And even Plato acknowledged a certain necessary love motivating knowledge. Faith in reason is narrower, to be sure, than faith in Christ, but as the heart encompasses the intellect in the knowing of truth, so, too, the faith in reason may be at the service of faith in Christ without diminishing the intellect and its own asketical rigor.
While we would not identify the quality of faith in reason with that of faith in Christ, nor the objects of each respective faith, we can, at least, I think identify a common stretching forth of the heart and the intellect centered within it to that reality external to the knower which draws by love the respective faculties into a communion of knowledge, mind to mind, thought to thought, person to person, and heart to heart. In this common movement of faith, the intellective knower and the lover of God are drawn to both know and be known.
If then faith has these common elements, if the heart brings together the unity of the person in the location of all sure knowledge, then we may after all begin to delimit something like a Christian philosophy, or at least outline its project.