NOTES for Co2 6:18
Relations are very different: professor - student, master - Servant, finally, just colleagues or friends. What distinguish the relations "father - son"? Not in the fact that one is more elder, and the other younger, - it's like that even in other forms. But the father and the son are linked in blood, by parentage, something common inside and essential. Such unity is initially in God: the unity of the Father and the Son in the Holy Spirit, and the Christ - Son of God is the Only-begotten Son, the unique, to whom God's relationship belongs by nature.
But the Son of God is at the same time the Son of Man, He unites through Himself the Divine nature and the creative nature, that is why opens through Him in us the way to be son of God. On the Pentecost day, the Holy Spirit descends powerfully into this world, and come true the predictions of the Old Testament, of which we read one of them on verse 16: " I will dwell in them, and walk in them ", - Therefore people become inside involved to God upon dwelling in them of the Holy Spirit, making like unto the Son of God. That is why Paul calls the Holy Spirit in another place Spirit of adoption, thanks to Whom we can address God: "? Abba! Father!"