Matthew 3:1-2.
“In those days comes John the Baptist, preaching in the wilderness of Judæa, and saying, Repent ye: for the kingdom of Heaven is at hand.”
How “in those days”? For not then, surely, when He was a child, and came to Nazareth, but thirty years after, John comes; as Luke also testifies. How then is it said, “in those days”? The Scripture is always wont to use this manner of speech, not only when it is mentioning what occurs in the time immediately after, but also of things which are to come to pass many years later. Thus also, for example, when His disciples came unto Him as He sat on the Mount of Olives, and sought to learn about His coming, and the taking of Jerusalem: and yet ye know how great is the interval between those several periods. I mean, that having spoken of the subversion of the mother city, and completed His discourse on that subject, and being about to pass to that on the consummation, he inserted, “Then shall these things also come to pass;” not bringing together the times by the word then, but indicating that time only in which these things were to happen. And this sort of thing he does now also, saying, “In those days.” For this is not put to signify the days that come immediately after, but those in which these things were to take place, which he was preparing to relate.
“But why was it after thirty years,” it may be said, “that Jesus came unto His baptism”? After this baptism He was thenceforth to do away with the law: wherefore even until this age, which admits of all sins, He continues fulfilling it all; that no one might say, that because He Himself could not fulfill it, He did it away. For neither do all passions assail us at all times; but while in the first age of life there is much thoughtlessness and timidity, in that which comes after it, pleasure is more vehement, and after this again the desire of wealth. For this cause he awaits the fullness of His adult age, and throughout it all fulfills the law, and so comes to His baptism, adding it as something which follows upon the complete keeping of all the other commandments.
To prove that this was to Him the last good work of those enjoined by the law, hear His own words: “For thus it becomes us to fulfill all righteousness.” Now what He says is like this: We have performed all the duties of the law, we have not transgressed so much as one commandment. Since therefore this only remains, this too must be added, and so shall we “fulfill all righteousness.” For He here calls by the name of “righteousness” the full performance of all the commandments.
Source: Homilies on the Gospel of St. Matthew (New Advent)