The sacred humanity of Christ was the connecting link between heaven and earth. The voice from heaven which declared Him to be the Beloved Son of the Eternal Father was not announcing a new fact or a new Sonship of Our Blessed Lord. It was merely making a solemn declaration of that Sonship, which had existed from all eternity, but which was now beginning to manifest itself publicly as Mediator between God and man. The Father’s good pleasure, in the original Greek, is recorded in the aorist tense, to denote the eternal act of loving contemplation with which the Father regards the Son.
The Christ Who came out of the water, as the earth had come out of the water at creation and after the Flood, as Moses and his people had come out of the waters of the Red Sea, was now glorified by the Holy Spirit appearing in the form of a Dove. The Spirit of God never appears in the figure of a Dove anywhere save here. The Book of Leviticus mentions offerings which were made according to the economic and social position of the giver. A man who could afford it would bring a bullock, and a poorer man would offer a lamb; but the poorest of all had the privilege of bringing doves. When the mother of Our Lord brought Him to the temple, her offering was a dove. The dove was the symbol of gentleness and peacefulness, but above all it was the type of sacrifice possible to the lowliest people. Whenever a Hebrew thought of a lamb or a dove, he immediately thought of a sacrifice for sin. Therefore, the Spirit descending upon Our Lord was for them a symbol of submission to sacrifice. Christ had already united Himself symbolically with man in baptism, in anticipation of His submergence into the waters of suffering; but now He was also crowned, dedicated, and consecrated to that sacrifice through the coming of the Spirit. The waters of the Jordan united Him with men, the Spirit crowned Him and dedicated Him to sacrifice, and the Voice attested that His sacrifice would be pleasing to the Eternal Father.
The seeds of the doctrine of the Trinity which were planted in the Old Testament began here to unfold. – from Life of Christ (1958)