Kindly tell them I'm not interested. "I'm happily atheistic, thank you very much." Most of the times, they leave.
If they don't, "If God is as you say, I don't need to be part of your church, nor believe in him. He'll love me regardless, and I'll be up in heaven with you guys. Try again when we meet up there."
If they then still insist... door, meet Jehovah's. Jehova's, take a step back. Or the other way around, if you're feeling particularly kind that day, and wish to give them a warning.