What reasons does Christianity give for "Why There Must Be A God?" I realize this question could also be addressed on Philosophy.SE, but I am very interested to get specifically the Christian perspective on this issue.

I was raised Catholic, but have left the Church in the last few years because of conflicts between my beliefs and the Church's. My lack of belief has definitely negatively affected my relationships with my family and I would really like a reason to believe once more. I suppose my real problem is that I have a hard time seeing a scenario, given what we have learned about the universe, that requires God to exist.

The question I would like to pose is, "According to Christianity, why must there be a God?"
There are several classical arguments for believing in God, including:

The cosmological argument, which says that (logically) everything exists because it is caused by something else or because it has always existed. It says that everything that has been caused by something else must be caused by something else and so on until you get to something that is without cause. A thing without cause must always have existed. The first cause is God.

The teleological argument argues that the universe looks as if it has a design and a purpose. It argues that this is because it has a designer who gave it a purpose. It asks for a reason for the order in the universe, for natural laws and principles, and finds an explanation in God.

The ontological argument argues that God must exist if you can imagine that He exists... I'm not sure why, to be honest.

The moral argument says that naturalistic explanations of morality, love etc. are inadequate to explain the existence of morality, love etc.

Each of these arguments has received considerable attention by philosophers and theologians, with counter-arguments and counter-counter arguments having been debated for centuries. At the very least, however, some version of these arguments may remove intellectual barriers to faith and position you to accept the possibility that God is real.

Of course, even if you accept the veracity of one of these arguments, the God in its conclusion isn't necessarily the God of Christianity. Nevertheless, the conclusion of these arguments does appear to be compatible with Christianity. Moreover, some writers suggest that the cumulative effect of the arguments is to validate the Christian world-view.

For most Christians, rational argument is only a part of why we believe. Instead, we choose Jesus on the basis of religious experience or because he fills an emotional or pragmatic need. In my view, nobody can be certain of God's existence (or non-existence), but you can look at the balance of probabilities. As such, we each have to take responsibility to look at the arguments in the light of our experience and then decide for ourselves what to believe.