Why am I certain that Jesus is God? Or that the Bible is true? Partly, just by faith. At least, that's what I would have answered ten years ago, basing my answer on my personal experience of reading the Bible as a teenager and sensing deep in my spirit that it was, in fact, true.
But an answer so subjective makes truth relative, validated by an individual's experience, desires and feelings rather than hard evidence. I once had a pastor who said (from the pulpit, no less!) "No one can argue with your experience," and I believe most people in our society bank on that very supposition. We assume our experience is authoritative, and therefore immune from questioning or examination. But if one person's experience says one thing, and another person's experience says quite the opposite, how can both be true? Both, admittedly, could be false, but there's only one true answer.
So, to properly defend my position that Christianity is the one true worship of the one true God, I first appeal to the way courts judge evidence. Two or more witnesses are required to substantiate that a matter is true.
The requirement of two or more witnesses brings me to the various accounts of Jesus' resurrection from the dead. Many people--some of whom had been at the cross three days before He rose--had interactions with Him. They ate with Him, they touched His nail-scarred hands, they felt His breath and had conversations with Him. At one point, He appeared to over 500 men (see 1 Corinthians 15:7), many of whom were still available to be cross-examined when Paul wrote that letter.
The case is strengthened by contrasting the absolute cowardice of the disciples at Jesus' arrest and immediately after His crucifixion with the unmitigated boldness after He rose. Ten of the eleven (Judas, remember, committed suicide before the resurrection) suffered brutal executions for refusing to agree that they had fabricated the resurrection story, and John was exiled to a prison camp. Why would these otherwise cowardly men willingly suffer for a lie?
The evidence for the resurrection of Jesus Christ is simply the starting point for explaining why I'm convinced that Christianity is true. And I could go more in depth to demonstrate the evidences, though doing so would take several blog posts. Beyond the resurrection, I'd reason that the event authenticates everything Jesus said about Himself. And from there, I could show reasons for believing that the Bible is reliable as God's Word.
Many may, of course, reject whatever evidence I might offer, and that's up to them. But they'd have to admit that my faith stands grounded in more than my subjective personal experience.