34 Now, why the number seventy-seven should contain all sins which are remitted in Baptism, there occurs this probable reason, for that the number ten implies the perfection of all righteousness, and blessedness, when the creature denoted by seven cleaves to the Trinity of the Creator; whence also the Decalogue of the Law was consecrated in ten precepts. Now the “transgression” of the number ten is signified by the number eleven; and sin is known to be transgression, when a man, in seeking something “more,” exceeds the rule of justice.
And hence the Apostle calls avarice “the root of all evils.” And to the soul which goes a-whoring from God, it is said, in the Person of the same Lord, “You were in hope, if you departed from Me, that you would have something more.” Because the sinner then has in his transgression, that is, in his sin, regard to himself alone— in that he wishes to gratify himself by some private good of his own (whence they are blamed “who seek their own, not the things which are Jesus Christ's;” and charity is commended, “which seeks not her own”); therefore, this number eleven, by which transgression is signified, is multiplied, not ten times, but seven, and so makes up seventy-seven.
For transgression looks not to the Trinity of the Creator, but to the creature, that is, to the man himself, which creature the number seven denotes. Three, because of the soul, in which there is a kind of image of the Trinity of the Creator (for it is in the soul that man has been made after the image of God); and four, because of the body. For the four elements of which the body is made up are known by all. And if any one know them not, he may easily remember, that this body of the world, in which our bodies move along, has, so to say, four principal parts, which even Holy Scripture is constantly making mention of, East, and West, and North, and South.
And forasmuch as sins are committed either by the mind, as in the will only, or by the works of the body also, and so visibly; therefore the Prophet Amos continually introduces God as threatening, and saying, “For three and four iniquities I will not turn away,” that is, I will not dissemble My wrath. Three, because of the nature of the soul; four, because of that of the body; of which two, man consists.
Source: Sermons on Selected Lessons of the New Testament (New Advent)