24 By the mystery of the divine nature we are forbidden to separate the birth of the living Son from His living Father. The Son of God suffers no such change of kind, that the truth of His Father's nature does not abide in Him. For even where, by the confession of one God only, He seems to disclaim for Himself the nature of God by the term “only,” nevertheless, without destroying the belief in one God, He places Himself in the unity of the Father's nature. Thus, when the Scribe asked Him, which is the chief commandment of the law, He answered, Hear, O Israel, the Lord our God is one Lord: you shall love the Lord your God with all your heart, and with all your soul, and with all your spirit, and with all your strength.
This is the first commandment. And the second is like it, You shall love your neighbour as yourself. There is none other commandment greater than these. They think that He severs Himself from the nature and worship of the One God when He pronounces as the chief commandment, Hear, O Israel, the Lord our God is one Lord, and does not even make Himself the object of worship in the second commandment, since the law bids us to love our neighbour, as it bids us to believe in one God.
Nor must we pass over the answer of the Scribe, Of a truth you have well said, that God is one, and there is none other but He: and to love Him with all the heart, and all the strength and all the soul, and to love his neighbour as himself, this is greater than all whole burnt offerings and sacrifices. The answer of the Scribe seems to accord with the words of the Lord, for He too proclaims the innermost and inmost love of one God, and professes the love of one's neighbour as real as the love of self, and places love of God and love of one's neighbour above all the burnt offerings of sacrifices. But let us see what follows.
Source: On the Trinity (New Advent)