2 The Son of God, our Lord Jesus Christ, has taught us a Prayer; and though He be the Lord Himself, as you have heard and repeated in the Creed, the Only Son of God, yet He would not be alone. He is the Only Son, and yet would not be alone; He has vouchsafed to have brethren. For to whom does He say, “Say, Our Father, which art in heaven?” Whom did He wish us to call our Father, save His own Father? Did He grudge us this? Parents sometimes when they have gotten one, or two, or three children, fear to give birth to any more, lest they reduce the rest to beggary.
But because the inheritance which He promises us is such as many may possess, and no one be straitened; therefore has He called into His brotherhood the peoples of the nations; and the Only Son has numberless brethren; who say, “Our Father, which art in heaven.” So said they who have been before us; and so shall say those who will come after us. See how many brethren the Only Son has in His grace, sharing His inheritance with those for whom He suffered death. We had a father and mother on earth, that we might be born to labours and to death: but we have found other parents, God our Father, and the Church our Mother, by whom we are born unto life eternal.
Let us then consider, beloved, whose children we have begun to be; and let us live so as becomes those who have such a Father. See, how that our Creator has condescended to be our Father!
Source: Sermons on Selected Lessons of the New Testament (New Advent)