Dominum et Vivificantem §21

²¹ That which during the theophany at the Jordan came so to speak "from outside," from on high, here comes "from within," that is to say from the depths of who Jesus is. It is another revelation of the Father and the Son, united in the Holy Spirit. Jesus speaks only of the fatherhood of God and of his own sonship-he does not speak directly of the Spirit, who is Love and thereby the union of the Father and the Son. Nonetheless what he says of the Father and of himself-the Son-flows from that fullness of the Spirit which is in him, which fills his heart, pervades his own "I," inspires and enlivens his action from the depths. Hence that "rejoicing in the Holy Spirit." The union of Christ with the Holy Spirit, a union of which he is perfectly aware, is expressed in that "rejoicing," which in a certain way renders "perceptible" its hidden source. Thus there is a particular manifestation and rejoicing which is proper to the Son of Man, the Christ-Messiah, whose humanity belongs to the person of the Son of God, substantially one with the Holy Spirit in divinity. In the magnificent confession of the fatherhood of God, Jesus of Nazareth also manifests himself, his divine "I"- for he is the Son "of the same substance," and therefore "no one knows who the Son is except the Father, or who the Father is except the Son," that Son who "for us and for our salvation" became man by the power of the Holy Spirit and was born of a virgin whose name was Mary.