Excuse me while I pad Disputations by cutting and pasting the following, which I emailed to an atheist who asked me the opening question.
Why are you a Christian?
Because of God's grace, of course.
I don't expect you'll find that a very satisfying answer, but it is the literal truth: God's grace is the cause of my accepting what He says about Himself to be true.
As for the natural reasons upon which this grace builds, I was raised a Roman Catholic and quickly outgrew the sophomoric atheism I tried on in college. The more I learn about what the Church teaches, the more reasonable I see it to be, the more sense it makes, the more questions it answers, the better answers it provides to those questions.
Which brings me back to my first statement, on God's grace. Human reason can tell someone what the Church teaches, it can examine the arguments for validity and soundness, it can apply philosophy to determine to some extent what must be true and what can't be true, it can determine whether a particular religious belief is reasonable.
But "reasonable" here does not mean "likely" or "probable" or "I'll take that bet." It means "in accord with reason," not "provable from reason"; the difference between the two is a distinction a lot of evangelical atheists fail to make.
Apart from what can be shown must be true or can't be true, human reason cannot determine what is true, any more than a tape recorder can determine the size of the person speaking into it.
So though I do have reasons for why I believe this or that religious or moral proposition is true – reasons that can be stated and debated, judged for soundness or persuasiveness –fundamentally, Christianity is a matter of faith. Faith is neither a subcategory of nor contrary to reason, just as hearing is neither a subcategory of nor contrary to sight.
There are what I'd call reasons for why I have faith in God, and through Him in His Church, but these reasons are not arguments whose conclusion is, "Therefore, Catholicism is the one true faith." They are, more properly, reasons for why my faith in God is reasonable.
Faith that can be demonstrated from reason is not faith. The atheist and the Christian agree that the Christian faith cannot be demonstrated from reason, but the Christian never said it could, and the atheist never proved it had to.