A patient man has great understanding, but a quick-tempered man promotes folly. — Proverbs 14:29

He that is slow to wrath is of great understanding. That's not natural wisdom—it's the fruit of knowing a God who is slow to wrath Himself, abounding in mercy, patient toward sinners. Christ bore the full brunt of divine wrath in our place, so we're free to follow His example in long-suffering. When we're quick to anger we're actually forgetting the Gospel—we've been forgiven everything, so we can afford to be patient with others.