In the gospel today we get the famous passage of how many time we must forgive. It's Matthew today so just gives us not 7 but 77.
Here it helps to know the background. In the rabbinic law 3, a religiously significant number, was the maximum required. The disciples then think they are being generous ask about 7, the perfect number. Jesus chooses 77.
To find that number you have to go all the way back to Genesis 4, and a descendent of Cain. Lamech is the very embodiment of wrath and vengeance. He says to his wives:
I have killed a man for wounding me, a young man for bruising me. If Cain is avenged seven times, then Lamech seventy-seven times.
Jesus calls us to the opposite. It is not enough to keep the rabbinic law. We are to be the mirror image of Lamech. As fiercely as he embodies anger and vengeance we must embody love and forgiveness.