1 You have heard what the Lord God, Jesus Christ, the Only Son of God, born of God the Father without any mother, and born of a Virgin mother without any human father, said, “I and My Father are One.” Receive this, believe it in such wise that you may attain to understand it. For faith ought to go before understanding, that understanding may be the reward of faith. For the Prophet has said most expressly, “Unless you believe, you shall not understand.” What then is simply preached is to be believed; what is with exactness discussed, is to be understood.
At first then to imbue your minds with faith we preach to you Christ, the Only Son of God the Father. Why is added, “The Only Son”? Because He whose Only Son He is, has many sons by grace. All the rest then, all saints are sons of God by grace, He Alone by Nature. They who are sons of God by grace are not What the Father is. And no saint has ever dared to say, what that Only Son says, “I and My Father are One.” Is He not then our Father too? If He be not our Father, how say we when we pray, “Our Father, which art in heaven”? But we are sons whom He has made sons by His Own will, not begotten as sons of His Own Nature.
And in truth He has begotten us too, but as it is said, as adopted ones, begotten by the favour of His adoption, not by Nature. And this too are we called, for that “God has called us into the adoption of sons;” we are though adopted, men. He is called the Only Son, the Only Begotten, in that, He is That which the Father is; but we are men, The Father is God. In then that He is That which the Father is; He said, and said truly, “I and My Father are One.” What is, “are One”? Are of one Nature. What is, “are One”? Are of one Substance.
Source: Sermons on Selected Lessons of the New Testament (New Advent)