Only Son: In the Old Testament, the title “son of God” is applied to heavenly beings: that is, angels (Gn 6:2; Ps 29:1). This has probably to do with the primitive conception of heavenly beings as members of the divine and superhuman order of beings. The title is at times applied to the people of Israel as a whole, expressing the close relationship of Yahweh with Israel because of the covenant (Ex 4:22; Hos 2:1). When the title is later used for an individual Israelite—as in the case of a Davidic king—it signifies God’s peculiar love and care for him (2 Sm 7:14; Ps 2:7).
In the New Testament, the title is almost always applied to Jesus. It hardly reflects the Old Testament background, since “Son of God” is not a messianic title in Judaism. Rather, it expresses the absolutely unique character of Jesus. Jesus acts as one having an intimate knowledge of God, and with God’s divine authority. He addresses God as “Abba, Father!”
In John, the title appears 23 times—the most in any New Testament writing. It expresses the intimate union of the Son with the Father, in virtue of which Jesus receives from the Father the power to judge and to confer life (Jn 5:19-23). In the Gospel, Jesus refers to God giving his “only Son.” This is to be seen as a soteriological rather than a metaphysical title. There is here an implicit reference to the story of Abraham, who was commanded to take his only son Isaac, whom he loved, and offer him as a holocaust to the Lord (Gn 22:2, 12). While God spared Abraham the sacrifice of his son, he does not spare his own. This is the greatest sign of God’s love for the world.