While American Christianity may seem to be waning, Christianity as a whole continues to grow. The Kingdom of God cannot be thwarted.
Luke 13:18 He said therefore, “What is the kingdom of God like? And to what shall I compare it? 19 It is like a grain of mustard seed that a man took and sowed in his garden, and it grew and became a tree, and the birds of the air made nests in its branches.”
Hat Tip: Wayne (who pillaged it from someone else)