Father Cassilly further elaborates on adoptive sonship. He explains: “Adoptive sonship amongst men requires, as we have seen, a common nature between father and child, and it must exist before the adoption; whereas, before our Divine adoption we have not this similarity with God, we acquire it only by the infusion of grace, which causes us to participate in the Divine nature. By grace, then, we become God’s adopted children, receiving a similarity of nature with Him, and we are gratuitously, that is without merit on our part, accepted into His family, dowered with a father’s love and affection and given a right to our heavenly inheritance in the next life. Is anything still wanting to enable us to say, ‘Abba, Father’?”
“Not only are we called adoptive children of God, but in many places of Scripture we are simply styled sons or children without any apparent qualification or restriction. ‘Of His own will hath He begotten us,’ says St. James (Jas 1:18), in explanation of which St. Thomas [Aquinas] remarks, ‘an adoptive son is sometimes said to be begotten on account of the spiritual generation, which is gratuitous and not natural.'”
“When Nicodemus came by night, Christ taught him that man must be born again of the Holy Ghost. The ruler in Israel, sorely puzzled at this teaching, asked how a man could be born a second time, and received the reply, ‘that which is born of the Spirit is spirit’ (Jn 3:6). St. John, who recorded this interview with Nicodemus, afterwards repeats the same doctrine: ‘Whosoever believeth that Jesus is the Christ, is born of God. And everyone that loveth Him Who begot, loveth him also who is born of Him’ (1 Jn 5:1).”
“In all these places of Holy Writ and numerous others, insistence is laid on a new birth of man by which he is born of God and becomes His offspring through the action of the Holy Spirit. This is not easily intelligible to the human mind, as was evident in the case of Nicodemus, who, though skilled in the religious teaching of his day, found it hard to comprehend. The difficulty is increased by the fact that our sonship of God is not now apparent, as we learn from St. John, and that our finished resemblance to Him will only be perfected in the vision of glory.”
Quotations from Francis Cassilly, A Story of Love, 2d ed. (St. Louis: B. Herder, 1917).