... just as Abraham “believed God, and it was counted to him as righteousness” (Galatians 3:6 ESV).
To believe in God as Abraham did is to be right with God because faith honors God. Faith says to God: "I believe what you say."
... Men fast, pray, watch, suffer. They intend to appease the wrath of God and to deserve God's grace by their exertions. But there is no glory in it for God, because by their exertions these workers pronounce God an unmerciful slave driver, an unfaithful and angry Judge. They despise God, make a liar out of Him, snub Christ and all His benefits; in short they pull God from His throne and perch themselves on it.
Faith truly honors God. And because faith honors God, God counts faith for righteousness.
Christian righteousness is the confidence of the heart in God through Christ Jesus. Such confidence is accounted righteousness for Christ's sake. Two things make for Christian righteousness: Faith in Christ, which is a gift of God; and God's acceptance of this imperfect faith of ours for perfect righteousness. Because of my faith in Christ, God overlooks my distrust, the unwillingness of my spirit, my many other sins. Because the shadow of Christ's wing covers me I have no fear that God will cover all my sins and take my imperfections for perfect righteousness.
God "winks" at my sins and covers them up. God says: "Because you believe in My Son I will forgive your sins until death shall deliver you from the body of sin."
Learn to understand the constitution of your Christian righteousness. Faith is weak, but it means enough to God that He will not lay sin to our charge. He will not punish nor condemn us for it. He will forgive our sins as though they amount to nothing at all. He will do it not because we are worthy of such mercy. He will do it for Jesus' sake in whom we believe.
Taken from Martin Luther's Commentary on Galatians.