St. Augustine has a comment on the Gospel for today’s Holy Mass (in the Novus Ordo), which concerns the number of times we must forgive those who sin against us. He works with the significance of numbers and brings in an interesting point of comparison. This sermon was preached in Milev in late 408 or early 409. Here is some of s. 83:
5. So what’s the meaning of seventy-seven times? Listen, brothers and sisters, it’s a great mysterious riddle, a marvelous kind of sacramental sign. When the Lord was baptized, the holy evangelist Luke here recounts his ancestry, the order, the series, the pedigree by which it eventually came to the birth of Christ Himself. Matthew began with Abraham and got to Joseph, coming down the generations. But Luke began counting by going up them. Why did the former come down, the latter go up? Because Matthew was drawing our attention to the birth of Christ, and how He came down to us; that’s why it was when Christ was born that he started coming down the generations to count them.
Luke, on the other hand, began counting them when Christ was baptized; that was the beginning of His Ascension, or going back up to God. So Luke began his count by going up and back, and he completed his count in seventy-seven generations. Who did he count from? Notice who from. He began counting from Christ, and went as far as Adam himself, who was the first to sin, and begot us under the obligations of sin. He went right back to Adam, and there are seventy-seven generations to count; and from Adam down to Christ seventy-seven. So if no generation has been left out, no sin is passed over where pardon should be withheld. Thus the reason he listed seventy-seven generations of His ancestry, the same number as the Lord mentioned in connection with forgiving sins, is that he started counting from baptism, in which all sins are undone.
Augustine is right about the number 77 insofar as the list of generations is concerned. He doesn’t seem to want to take into consideration Genesis 4:24, where Lamech speaks of seventy-seven-fold vengence to be exacted compared to only seven-fold venegence for Cain. Peter, in the Gospel passage, wanted to overturn the law of Cain, but Jesus went way beyond that, saying that vengence itself much be avoided. Augustine missed this point, it seems. Keep in mind as well that when Augustine speaks of a sacramental sign here, he is not talking about the sacraments in the sense that we usually think of them now. This refers to the hidden significance in the events of the life of the Lord. The Baptism of the Lord by John began His public ministry. This ministry ends with the Ascension. Either way, the Doctor of Grace comes to the same conclusion: all sins are to be forgiven.