[The] state must always water down the absolute quality of Jesus’ commands and turn them into a form of external morality — precisely the opposite of the gospel of grace. Jacques Ellul goes so far as to say the New Testament teaches no such thing as a “Judeo-Christian ethic.” It commands conversion and then this, “Be perfect … as your heavenly Father is perfect.” Read the Sermon on the Mount and try to imagine any government enacting that set of laws.
A state government can shut down stores and theaters on Sunday, but it cannot compel worship. It can arrest and punish KKK murderers but cannot cure their hatred, must less teach them love. It can pass laws making divorce more difficult but cannot force husbands to love their wives and wives their husbands. It can give subsidies to the poor but cannot force the rich to show them compassion and justice. It can ban adultery but not lust, theft but not covetousness, cheating but not pride. It can encourage virtue but not holiness. – Philip Yancey, What’s So Amazing about Grace?