Critics of Christianity claim that the definition of faith is when you believe in something that can't be proved. They say that we're trusting in a story about what's going to happen after we die, but we won't know anything for sure until that happens, and there's no way to prove it before then.
But that's not what faith is according to the Bible! For starters, when we look at the majesty of a starry night sky, the beauty of a sunset, the delicate ingenuity of a hummingbird in flight, or the inscrutable complexity of the cells that make up our body, we learn about the nature of the one who made nature: His majesty, His vast intelligence, his care and concern for the littlest things, and the undeniable order from which everything seems determined to pull away. This is not a universe created by a chaotic, capricious being, but the handiwork of a loving, careful, intelligent God.
But we see more than just the aspects of His character revealed through His creation. We also feel conviction for our sin. Even those who are unaware of the existence of Jesus or the Bible still feel that tug inside our souls, like a gentle magnet reminding us where the line between right and wrong is, like an indelible stain on our insides when we cross that line. And every time we do sin and yet aren't immediately inflicted with the judgement we know we deserve, we see evidence of the love, mercy, and grace He has for us.
Finally, most importantly, we see Jesus. God Himself, born as one of us, living among us, teaching us, healing us, even feeding us, and finally, dying a terrible death to free us from the penalty of what we've done. After that He rose from the dead, to show us all what would one day happen to everyone who trusts in Him. We are not destined to spend eternity as spirits playing stringed instruments in clouds; we will be raised as He was in bodies of flesh, and yet somehow devoid of the corruption that tempted us to sin, never again to become weak, sick, tired, or old.
Our faith as Christians is not a matter of believing what will happen when we die, but of belief in the nature and the character of the one who makes the promise. It is grounded in the observable evidence of Jesus and the life He lived.