6 And, Brethren, observe in this a yet greater mystery. In the number seventy-seven is a mystery of the remission of sins. So many are the generations found to be from Christ to Adam. Now then, ask with somewhat more careful diligence for the secret meaning of this number, and enquire into its hidden meaning; with more careful diligence knock, that it may be opened unto you. Righteousness consists in the observance of the Law of God: true. For the Law is set forth in ten precepts.
Therefore it was that the servant in the parable “owed ten thousand talents.” This is that memorable Decalogue written by the finger of God, and delivered to the people by Moses, the servant of God. He “owed” then “ten thousand talents;” which signifies all sins, with reference to the number of the Law. And the other “owed a hundred denarii;” derived equally from the same number. For a hundred times a hundred make ten thousand; and ten times ten make a hundred. And the one “owed ten thousand talents,” and the other ten times ten denarii.
For there was no departure from the number of the law, and in both numbers you will find every kind of sin included. Both are debtors, and both implore and beg for pardon; but the wicked, ungrateful servant would not repay what he had received, would not grant the mercy which had been undeservedly accorded to him.
Source: Sermons on Selected Lessons of the New Testament (New Advent)