The LORD is compassionate and gracious, slow to anger, abounding in love ... he does not treat us as our sins deserve (Psalm 103:8 and 10).
'God's anger is different from ours. We are quick to anger, we make people pay who have wronged us, and then, nonetheless, we nurse our grievances. God is slow to anger, provides for our forgiveness, and then remembers our sins no more (Keller).
Keller points out that the above verse (Psalm 103:8), is a quote from Exodus (34:6). In the Exodus text Moses says that God 'does not leave the guilty unpunished.' How can Moses say the guilty will not go unpunished and yet David write that 'he does not treat us as our sins deserve'? The answer, of course, is the cross. The cross shows us how much it cost God to punish sin without punishing us. That's why this Psalm can speak of an infinite love as high as the heavens are above the earth, and a perfect forgiveness that removes our rebellions as far as the east is from the west.