(Except for first and third parts, from "Supernatural Adoption,"Catholic Encyclopedia: http://www.newadvent.org/cathen/01148a.htm)
God, the fullness of Being, is everything we want. He is also our Father when we are in a state of grace. Filiation entails the offspring's mirroring the parent. Though we clearly do not deserve it, we have been chosen as His children in the same way a master can make any of his slaves his own son. He has gone beyond the threshold of justice by adopting us, strangers, as His own children and heirs.
The saints in the days of the Old Law were also adopted children of God. Adoption is inseparable from sanctity. Such sayings as those of Exodus (4:22), "Israel is my son, my firstborn", Osee (1:10), "Ye are the sons of the living God", and Romans (9:4), "Israelites to whom belongeth the adoption as of children", are spoken of God's chosen people taken collectively.
It is in the New Testament, which marks the fullness of time and the coming of the Redeemer, that the revelation of this heaven-born privilege is completed (cf. Galatians 4:1). "Son of God" is an expression of no infrequent use in the Synoptic Gospels, and as therein employed, the words apply both to Jesus and to ourselves;. Surely in our case it cannot of itself afford us a sufficiently stable foundation on which to establish a valid claim to adopted sonship. As a matter of fact, when St. Matthew (v, 9, 45) speaks of the "children of God", he means the peacemakers, and when he speaks of "children of your Father who is in Heaven", he means those who repay hatred with love, thereby implying throughout nothing more than a broad resemblance to, and moral union with God. The charter of our adoption is properly recorded by St. Paul (Romans 8; Ephesians 1; Galatians 4); St. John (Prologue and I Epistle 1, 3); St. Peter (I Epistle 1); and St. James (I Epistle 1). According to these several passages we are begotten, born of God. He is our Father, but in such wise that we may call ourselves, and truly are, His children, the members of His family, brothers of Jesus Christ with whom we partake of the Divine Nature and claim a share in the heavenly heritage. This divine filiation, together with the right of co-heritage, finds its source in God's own will and graceful condescension. When St. Paul, using a technical term borrowed from the Greeks, calls it adoption, we must interpret the word in a merely analogical sense. In general, the correct interpretation of the Scriptural concept of our adoption must follow the golden mean and locate itself midway between the Divine Sonship of Jesus on the one hand, and human adoption on the other -- immeasurably below the former and above the latter. Human adoption may modify the social standing, but adds nothing to the intrinsic worth of an adopted child. Divine adoption, on the contrary, works inward, penetrating to the very core of our life, renovating enriching, transforming it into the likeness of Jesus, "the firstborn among many brethren".... There will ever be between our adoption and the filiation of Jesus the infinite distance which separates created grace from hypostatical union. And yet, that intimate and mysterious communion with Christ, and through Him with God, is the glory of our adopted sonship: "And the glory which thou hast given me, I have given to them -- I in them and thou in me" (John, xvii, 22, 23).
The oft-repeated emphasis which Holy Writ lays on our supernatural adoption won great popularity for that dogma in the early Church. Baptism became the occasion of a spontaneous expression of faith in our adopted sonship. St. Irenaeus (Adv. Haereses, iii, 17-19); St. Athanasius (Cont. Arianos, ii, 59); St. Cyril of Alexandria (Comment. on St. John, i, 13, 14); St. John Chrysostom (Homilies on St. Matthew, ii, 2); St. Augustine (Tracts 11 and 12 on St. John); St. Peter Chrysologus (Sermon 72 on the Lord's Prayer) -- all seem willing to spend their eloquence on the sublimity of our adoption. The Son is truly God, else how could He deify us? The Holy Ghost is truly God, else how could His indwelling sanctify us? The Incarnation of the Logos is real, else how could our deification be real? Be the value of such arguments what it may, the fact of their having been used, and this to good effect, bears witness to the popularity and common acceptance of the dogma in those days.
What is the essential factor or formal cause of our supernatural adoption? St. Thomas, pointed to habitual grace (an expression coined by Alexander) as the essential factor of our adopted sonship. For them the same inherent quality which gives new life and birth to the soul gives it also a new filiation. Says the Angel of the Schools (III:9:23, ad 3am), "The creature is assimilated to the Word of God in His Unity with the Father; and this is done by grace and charity. . . . Such a likeness perfects the idea of adoption, for to the like is due the same eternal heritage." (See GRACE.) This last view received the seal of the Council of Trent (sess. VI, c. vii, can. 11). "The Three Persons of the Blessed Trinity, the Father, the Son and the Holy Ghost", says St. Augustine (Tract 76; In Joan), "come to us as long as we go to Them, They come with Their help, if we go with submission. They come with light, if we go to learn; They come to replenish, if we go to be filled, that our vision of Them be not from without but from within, and that Their indwelling in us be not fleeting but eternal." And St. Paul (I Cor., iii, 16, 17), "Know you not that you are the temple of God and that the Spirit of God dwelleth in you? But if any man violate the temple of God, him shall God destroy. For the temple of God is holy, which you are."
In the gospels, Jesus compares His father with a good father who is prepared to divide his estate and to have it squandered by his sons. He also talks about the Merciful Father who welcomes the humble and contrite Prodigal Son who clamors,"Father, I have sinned against heaven and before You: I am no longer worthy to be called your son."; God, for his part, is always willing to take us back and even to tender a feast. In the sacrament of penance, he throws a party of grace for the penitent.
III. Divine filiation has several consequences.
It makes one have the confidence of a son of a king. St. Josemaria recalled such confidence in a university student: I was thinking about what you told me…. That I am a son of God. And I found myself walking down the street, head up and chin out, a feeling of pride within me. A son of God!…"
One takes pride in one's father. Children sometimes brag about their superman daddies. They boast, "My dad can lick your dad!" or "My dad makes S$2m a year. How much does yours make?" We Christians cannot be too proud of Our Great Daddy in heaven. He is being Himself.
One can turn to one's Daddy for help. Our Lord says that God governs the lilies of the field and the birds of the air; "the hairs of your head are all numbered." When our world seems to collapse on top of us, when we feel like scum, we just call out to Him.
If we are sunk in sin, even then we can beg the help of Our Father of mercies. Like the prodigal son, we can tell Him, "Lord… You entrusted me with this or that, and I put my trust in You. I know you are my Father…."
Filial piety demands our constant dealing with God in mental prayer, sacrifices at work ("Let me carry this sweet burden like Aeneas carrying his father Anchises") and petitions in the face of temptation or discouragement.
