Tertullian, On Prayer Chapter 2: The First Clause

The prayer begins with a testimony to God, and with the reward of faith, when we say, “Our Father who art in the heavens;” for (in so saying), we at once pray to God, and commend faith, whose reward this appellation is. It is written, “To them who believed on Him He gave power to be called sons of God.” However, our Lord very frequently proclaimed God as a Father to us; nay, even gave a precept “that we call no one on earth father, but the Father whom we have in the heavens:” and so, in thus praying, we are likewise obeying the precept. Happy they who recognize their Father! This is the reproach that is brought against Israel, to which the Spirit attests heaven and earth, saying, “I have begotten sons, and they have not recognized me.” Moreover, in saying “Father,” we also call Him “God.” That appellation is one both of filial duty and of power. Again, in the Father the Son is invoked; “for I,” saith He, “and the Father are One.” Nor is even our mother the Church passed by, if, that is, in the Father and the Son is recognized the mother, from whom arises the name both of Father and of Son. In one general term, then, or word, we both honour God, together with His own, and are mindful of the precept, and set a mark on such as have forgotten their Father.