On the second half of a proverb
“To err is human, to forgive divine.”
The first half gets quoted alone, usually by whoever erred. On its own it is an excuse, and Pope did not write it as one.
The pairing is the argument. Erring is what the species does by default, requiring no effort at all; forgiving is placed at the opposite end, among the things that do not come naturally to us. Divine is not praise here. It is a measure of distance.