One of the most basic beliefs of Christianity is this:
(G) God exists.
If claim (G) is false (or improbable), then Christianity is false (or improbable).
If claim (G) is true (or probable), that would not prove that Christianity is true (or probable), but it would provide some support for the truth of Christianity, and it would help the case for Christianity by eliminating some alternatives to Christianity (e.g. atheism and Secular Humanism).
Before we investigate the question "Does God exist?", we need to clarify the meaning of claim (G):
What Does the Claim "God Exists" Mean?
We also ought to formulate an approach to investigating this controversial question:
Investigating the Question "Does God exist?"
An investigation of this question should include both of the following kinds of considerations:
Reasons and Arguments For the Existence of God
Reasons and Arguments Against the Existence of God
After identifying and evaluating the best reasons and arguments for and against (G), one needs to synthesize the various pros and cons and arrive at an all-things-considered conclusion:
All Things Considered, What is the Probability of (G)?