It's awfully hard to improve on John 3:16. It's got it all: God's love, the price he was willing to pay, the gift he offers us.
For God so loved the world, that he gave his only begotten son, so that whoever believes in him will not perish, but have eternal life. (Janet's hodge-podge version.)
Shucks! All the good answers are already taken.
I really really really hope that's not your definition of Christianity, Roger.

hmm...Okay, we're all writers, so I dare you -- one sentence to express the essence of Christianity. Here's my attempt:
God is Love.
I'm going to cheat here because I can't improve on this.
"There is no pit so deep where God's love is not deeper still."
~ Corrie ten Boom
C