Roman Catechism 34.21

Chapter 34 — Part IV: Prayer — The Lord's Prayer: Introduction and Our Father Who Art in Heaven

Section 21. 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.