Taken from The Reason for God, by Tim Keller:
"If you are avoiding sin and living morally so that God will have to bless and save you, the ironically, you may be looking to Jesus as a teacher, model, and helper but you are avoiding him as Savior. You are trusting in your own goodness rather than in Jesus for your standing with God. You are trying to save yourself by following Jesus. That, ironically, is a rejection of the gospel of Jesus. It is a Christianized form of religion. It is possible to avoid Jesus as Savior as much by keeping all the Biblical rules as by breaking them...The devil, if anything, prefers Pharisees - men and women who try to save themselves. They are more unhappy than either mature Christians or irreligious people, and they do more spiritual damage."
I've never heard it said that way before, but it is dead on. The spirit of legalism is an ever present danger because it feeds on human pride, an ever present sin to be killed. The antidote to pride and legalism as a healthy does of biblical realism. We are wretched sinners, not fictitious sinners, who need real grace, not merely fictitious grace. Thus Luther's words to Melanchthon should be heard (and preached) again:
"If you are a preacher of grace, then preach a true and not a fictitious grace; if grace is true, you must bear a true and not a fictitious sin. God does not save people who are only fictitious sinners. Be a sinner and sin boldly, but believe and rejoice in Christ even more boldly, for he is victorious over sin, death, and the world. As long as we are here [in this world] we have to sin. This life is not the dwelling place of righteousness, but, as Peter says, we look for new heavens and a new earth in which righteousness dwells. It is enough that by the riches of God’s glory we have come to know the Lamb that takes away the sin of the world. No sin will separate us from the Lamb, even though we commit fornication and murder a thousand times a day. Do you think that the purchase price that was paid for the redemption of our sins by so great a Lamb is too small? Pray boldly—you too are a mighty sinner."