Catechism of the Council of Trent: "Our Father, who art in heaven"
God Is Called Father Because He Has Granted Us Redemption
The creation of the world and God's providence are, then, of great weight in bringing into relief the singular love of God for the human race and the special care He takes of man. But far above these two shines the work of redemption, so much so indeed that our most bountiful God and Father has crowned His infinite goodness towards us by granting us this third favour.
Accordingly the pastor should instruct his spiritual children and constantly recall to their minds the surpassing love of God for us, so that they may be fully alive to the fact that having been redeemed in a wonderful manner they are thereby made the sons of God. To them, says St. John, He gave power to be made the sons of God . . . and they are born of God.
This is why Baptism, the first pledge and token of our redemption, is called the Sacrament of regeneration; for it is by Baptism that we are born children of God: That which is born of the Spirit, says our Lord, is spirit; and: You must be born again. In the same way we have the words of St. Peter: Being born again, not of corruptible seed, but incorruptible, by the word of God who liveth.
By reason of this redemption we have received the Holy Ghost and have been made worthy of the grace of God. As a consequence of this gift we are the adopted sons of God, as the Apostle Paul wrote to the Romans when he said: Ye have not received the spirit of bondage again in fear, but you have received the spirit of adoption of sons, whereby we cry: "Abba, Father." The force and efficacy of this adoption are thus set forth by St. John: Behold what manner of charity the Father hath bestowed upon us, that we should be called, and should be the sons of God.
Duties We Owe Our Heavenly Father
These points having been explained, the faithful should be reminded of all they owe in return to God, their most loving Father, so that they may be aware of the extent of the love, piety, obedience and respect they are bound to render to Him who has created them, who watches over them, and who has redeemed them; and with what hope and trust they should invoke Him.
But to enlighten the ignorant and to correct the false ideas of such as imagine prosperity and success in life to be the only test that God preserves and maintains His love towards us, and that the adversities and trials which come from His hand are a sign that He is not well disposed towards us and that He entertains hostile dispositions towards us, it will be necessary to point out that even if the hand of the Lord sometimes presses heavily upon us, it is by no means because He is hostile to us, but that by striking us He heals us, and that the wounds coming from God are remedies.
He chastises sinners so as to improve them by this lesson, and inflicts temporal punishments in order to deliver them from eternal torments. For though He visits our iniquities with a rod and our sins with stripes, yet his mercy he will not take away from us.
The faithful, therefore, should be recommended to recognise in such chastisements the fatherly love of God, and ever to have in their hearts and on their lips the saying of Job, the most patient of men: He woundeth and cureth; he striketh and his hands shall heal; as well as to repeat frequently the words written by Jeremias in the name of the people of Israel: Thou hast chastised me and I was instructed, as a young bullock unaccustomed to the yoke: convert me and I shall be converted; for thou art the Lord my God; and to keep before their eyes the example of Tobias who, recognising in the loss of his sight the paternal hand of God raised against him, cried out: I bless thee, O Lord God of Israel, because thou hast chastised me and thou hast saved me.
In this connection the faithful should be particularly on their guard against believing that any calamity or affliction that befalls them can take place without the knowledge of God; for we have His own words: A hair of your heads shall not perish. Let them rather find consolation in that divine oracle read in the Apocalypse: Those whom I love I rebuke and chastise; and let them find comfort in the exhortation addressed by St. Paul to the Hebrews: My son, neglect not the discipline of the Lord; neither be thou weaned whilst thou art rebuked by him: for whom the Lord loveth he chastiseth, and he scourgeth every son whom he receiveth.... But if you be without chastisement, ... then are you bastards and not sons.... Moreover if we have had the fathers of our flesh for instructors, and we reverenced them, shall we not much more obey the Father of spirits and live?
"Our"
When we invoke the Father and when each one of us calls Him our Father, we are to understand thereby that from the privilege and gift of divine adoption it necessarily follows that all the faithful are brethren and should love each other as such: You are all brethren for one is your Father who is in heaven. This is why the Apostles in their Epistles address all the faithful as brethren.
Another necessary consequence of this adoption is that not only are the faithful thereby united in the bonds of brotherhood, but that, the Son of God being truly man, we are called and really are his brethren also. Thus, in his Epistle to the Hebrews the Apostle, speaking of the Son of God, wrote as follows: He is not ashamed to call them brethren, saying: "I will declare thy name to my brethren." And long before this, David had foretold this of Christ the Lord; while Christ Himself thus addresses the women in the Gospel: Go, tell my brethren that they go into Galilee; there they shall see me. These words, as we know, He pronounced only after His Resurrection and when He had already put on immortality, thus showing that no one is at liberty to imagine that the bonds of brotherhood with us have been severed by His Resurrection and Ascension into heaven. Not only has the Resurrection of Christ not dissolved this union and love, but we know that one day, when from His throne of glory and majesty He shall judge mankind of all ages, He will call even the very least of the faithful by the name of brethren.
Indeed, how can we be other than brethren of Christ, seeing that we are called His co-heirs? Doubtless He is the first begotten, the appointed heir of all things; but we are begotten in the second place after Him, and are His co-heirs according to the measure of heavenly gifts we receive and according to the extent of the charity by which we show ourselves servants and cooperators of the Holy Ghost. He it is who by His inspirations moves and inflames us to virtue and good works, in order that we may be strengthened by His grace valiantly to undertake the combat that must be waged to secure salvation. And if we wisely and firmly carry on this combat we shall at the close of our earthly career be rewarded by our heavenly Father with the just recompense of that crown promised and held out to all those who run the same course. God, says the Apostle, is not unjust that He should forget your work and love.