This Brennan Manning quote has become a common claim: “The greatest single cause of atheism in the world today is Christians: who acknowledge Jesus with their lips, walk out the door, and deny Him by their lifestyle. That is what an unbelieving world simply finds unbelievable.”
Never mind that he was living a double life himself of an alcoholic when he said this, giving his popular lectures and getting drunk, until it ruined his marriage and killed him – it is false on its own. It implies that if Christians were just more Christlike, the world would embrace the Church. That would mean being better than Jesus who told his relatives “the world cannot hate you” because you are indistinguishable from it.
The Church is not, and never has been, justified by its own superior holiness, nor is unbelief justified by observing signs of fallenness in the Church. In God’s own words to Habakkuk, “the just shall live by faith”.