Matthew 5:27-28.
“You have heard that it was said to them of old time, You shall not commit adultery; but I say unto you, that every one who looks upon a woman to lust after her, has committed adultery with her already in his heart.”
Having now finished the former commandment, and having extended it unto the height of self-denial, He, advancing in course and order, proceeds accordingly unto the second, herein too obeying the law.
“And yet,” it may be said, this is not the second, but the third; for neither is the first, “You shall not kill,” but “The Lord your God is one Lord.”
Wherefore it is worth inquiring too, why He did not begin with that. Why was it then? Because, had He begun from thence, He must have enlarged it also, and have brought in Himself together with His Father. But it was not as yet time to teach any such thing about Himself.
And besides, He was for a while practising His moral doctrine only, being minded from this first, and from His miracles, to convince the hearers that He was the Son of God. Now, if He had said at once, before He had spoken or done anything, You have heard that it was said to them of old time, “I am the Lord your God, and there is none other but me,” but I say unto you, Worship me even as Him; this would have made all regard Him as a madman. For if, even after His teaching, and His so great miracles, while not even yet was He saying this openly, they called Him possessed with a devil; had He before all these attempted to say any such thing, what would they not have said? What would they not have thought?
But by keeping back at the proper season His teaching on these subjects, He was causing that the doctrine should be acceptable to the many. Wherefore now He passed it by quickly, but when He had everywhere established it by His miracles, and by His most excellent teaching, He afterwards unveiled it in words also.
For the present, however, by the manifestation of His miracles, and by the very manner of His teaching, He unfolds it on occasion, gradually and quietly. For His enacting such laws, and such corrections of laws, with authority, would lead on the attentive and understanding hearer, little by little, unto the word of His doctrine. For it is said, “they were astonished at Him, because He taught not as their Scribes.”
Source: Homilies on the Gospel of St. Matthew (New Advent)