Here is why I believe I am a Christian: I believe I have a personal relationship with my Lord and Savior. I believe in the grace offered by the Resurrection. I believe that whatever spiritual rewards I may reap come directly from trying to live the example set by Christ. Whether or not I succeed in living up to that example is primarily between Him and me.
My understanding of Christianity is that it doesn't require me to prove my faith to anyone on this plane of existence. It is about a direct relationship with the divine and freely offered salvation. That's one of the reasons that when my generic "There must be something out there" gut feeling blossomed into a desire for a personal connection to that "something," it was Christianity that I choose to explore. They'll let anyone in.
To be clear, I don't just believe in God. I am a Christian. Decades of mass culture New Ageism has fluffed up "belief in God" into a spiritual buffet, a holy catch-all for those who want to cover all the numbers: Pascal's wager as a roulette wheel and not a coin toss. Me, I'm going all in with Jesus. It's not just that the payoff could be tremendous--it already has been! The only cost is the judgment that comes from others, from telling people that my belief has a specific shape, with its own human legacy of both shame and triumph.
This makes my day! There's almost nothing I like more than reading about people coming to Christ.