13 A careful inquiry will make it plain that it is not without some reason that this latter number is made to refer to the purging of all sins. For the number ten is shown to be, as one may say, the number of justice [righteousness] in the instance of the ten precepts of the law. Moreover, sin is the transgression of the law. And the transgression of the number ten is expressed suitably in the eleven; whence also we find instructions to have been given to the effect that there should be eleven curtains of haircloth constructed in the tabernacle; for who can doubt that the haircloth has a bearing upon the expression of sin?
Thus, too, inasmuch as all time in its revolution runs in spaces of days designated by the number seven, we find that when the number eleven is multiplied by the number seven, we are brought with all due propriety to the number seventy-seven as the sign of sin in its totality. In this enumeration, therefore, we come upon the symbol for the full remission of sins, as expiation is made for us by the flesh of our Priest, with whose name the calculation of this number starts here; and as reconciliation is also effected for us with God, with whose name the reckoning of this number is here brought to its conclusion by the Holy Spirit, who appeared in the form of a dove on the occasion of that baptism in connection with which the number in question is mentioned.
Source: The Harmony of the Gospels (New Advent)