John Piper on legalism: “It seems beyond doubt that God hates legalism as much as He hates alcoholism. And I believe it is a literal understatement that legalism has brought more people to eternal ruin than alcohol has, though the devastations of alcohol are huge.
“Let us not be deceived by outward appearances. Satan ‘disguises himself as an angel of light’ (2 Cor. 11:14). He keeps his deadliest diseases most sanitary. He clothes his captains in religious garments and houses his weapons in temples. Legalism is a more dangerous disease than alcoholism because it doesn’t look like one. Alcoholism makes men fail; legalism helps them succeed in the world. Alcoholism makes men depend upon the bottle; legalism makes them self-sufficient, depending on no one. Alcoholism destroys moral resolve; legalism gives strength. Alcoholics don’t feel welcome in the church; legalist love to hear their morality extolled in church.”
“Therefore, what we need in the church is not front-end regulations to try to keep ourselves pure. We need to preach and pray and believe that neither circumcision nor uncircumcision, neither teetotalism nor social drinking, neither legalism nor alcoholism is of any avail with God, but only a new heart.”
From Brothers, We Are Not Professionals.