In the development of Old Testament ideas, the theme of fraternity has a rather consistent development, in which the implications of being members of the same family must be taken into account, even when the texts do not always employ the terms sister or brother.
It would seem that the whole path of biblical revelation stretches out between two poles: it begins with Adam, the point of origin for a humanity bound by a communion of blood because it descends “from one man” (Acts 17:26; cf. Gen 1-2), and it ends in Jesus Christ, “the firstborn within a large family” (Rom 8:29), and therefore the only place of communion in which the original kinship between people has absolute fulfillment. This fraternity of all people in the Lord derives from the relationship of sonship that Jesus has with the Father, and indicates our new way of relating to God in Jesus Christ, that is, as children of the Father.
However, in the New Testament the nature of human existence raises a constitutive problem that lays bare the ambiguous way in which we are brothers and sisters. We are all children of Adam, the only progenitor of the first creation, and also children of God, because we are redeemed in Christ. One could say we are originally marked by a double paternity, from which follows a double title of fraternity, no less than an inner rupture. In reality, the paternity of God does not overlap with that of Adam, but goes back to God through Adam, “the son of God” (Luke 3:38).