The greatest single cause of atheism in the world today is Christians who acknowledge Jesus with their lips and walk out the door and deny Him by their lifestyle. That is what an unbelieving world simply finds unbelievable.

Brennan Manning
Brennan Manning (1934-2013) American author, laicized priest, theologian, speaker [Richard Francis Xavier Manning]

Widely attributed to Manning (1, 2, 3), though I cannot find an original citation.

See King.