God existing is something which I can't understand how an educated person can doubt. Not just the nice and elegant arguments for God (though some of those I believe to be undoubted). But the concept of so many things only work because it presupposes an eternal, maximal, prime, transcendental, good, and personal (that is, interested in you) God.
The thing that I don't have nearly as knock down is the reality of the Gospel. While I have my own reasons for believing in the testimony of the Gospels (and the epistles), not all of which are entirely empirical in nature, I do understand the sudden doubt that can wash over someone.
After reading Descartes, I have no idea how anyone could ever have an existential crisis. Since even recognizing you exist presupposes God. Many people know the cliff notes, but don't understand the implications. That the first thing you can know is that you exist, but buried inside of that is the recognition of necessity of God. Which implies that the first thing you can know is actually not that you exist, but that God is working on you.
If you are losing your faith in God, I just call that ignorant. Go and read the greats of philosophy or theology. Many people along different avenues have shown the necessity of God. So if you are a rationalist (like me) who trust his ability to form logical constructs almost more than any other part for being certain on something, there is plenty there. If you are someone who trust his ability to derive information from his senses, there are people for that too.
But losing your faith in Christ? The best advice I ever got: pray on it. Read scripture.