The ability of human couples to beget life is the path along which the history of salvation progresses. Seen this way, the couple’s fruitful relationship becomes an image for understanding and describing the mystery of God himself, for in the Christian vision of the Trinity, God is contemplated as Father, Son and Spirit of love. The triune God is a communion of love, and the family is its living reflection. Saint John Paul II shed light on this when he said, “Our God in his deepest mystery is not solitude, but a family, for he has within himself fatherhood, sonship and the essence of the family, which is love. That love, in the divine family, is the Holy Spirit”. The family is thus not unrelated to God’s very being. This Trinitarian dimension finds expression in the theology of Saint Paul, who relates the couple to the “mystery” of the union of Christ and the Church.An Analogy
A mother and father live together with their young son.
The father buys bread for his family.
His son is too young to feed himself, so the father helps him eat.
The mother doesn't need to be fed. She simply takes the bread to eat.
Over time, the father and the mother and the son are the bread. The bread is what sustains their bodies and forms new cells. The bread is analogous to the essence of the Trinity. Each person is the bread/essence.
The father is analogous to God the Father. In eternity, there is no origin of the divine essence, but God the Father is said to be the source of the Son and the Holy Spirit. So too the human father is the source of the bread that sustains his family.
The son is analogous to God the Son. God the Son receives His essence from God the Father. God the Father begets the Son. So too, the human son receives his sustenance from his human father.
The mother is analogous to God the Holy Spirit. The Holy Spirit proceeds from the Father and the Son. Procession is not the same as generation or begetting. In human terms, the only way we can distinguish modes of receiving is (1) passive reception or (2) active taking. God the Father and God the Son are never said to "generate" or "give" the divine essence to the Holy Spirit. The Holy Spirit is the only person of the Trinity who is said to act with respect to His own procession. So too, the human mother actively takes her sustenance from the father and son. (You can see how our Eastern Orthodox brothers and sisters might say just the father!)
Of course, no analogy is perfect. In this analogy, the bread (essence) is divided into shares among the family members. In contrast, the divine essence has no quantity and is indivisible. The divine essence also has no origin, unlike the bread in this example where the father purchases it.
Nevertheless, we can see in this family an ideal of human love that reflects eternal Love. The father gives. The son receives. The mother takes. There is no animosity or bitterness among them regarding who gets how much bread. The son does not resent that he is dependent on the father for his bread. The father and the son do not care how much bread the mother takes. The mother is not ashamed to take bread from the father and the son. What is theirs is hers. What is each of theirs is all of theirs.
The father gives all that he is to the son. The son receives all that he is from the father. The mother takes all that she is from both of them. This is love. This is the image of our God the Most Holy Trinity.