SONS OF GOD IN OLD AND NEW
While the term "sons of God" is used in both the Old and New Testaments, the sense in which they are used is quite different.
Qn: The title "sons of God" is not unique to the New Testament since it is also found in multiple contexts within the Old Testament. Therefore, what is so unique about believers in Christ being called the sons of God?
In the Old Testament, the title is used in three senses: celestial, corporate, and coronal. In the celestial sense, the heavenly beings or angels who meet with the Lord in heaven are called sons of God (Job 1:6; 2:1; 38:7). In the corporate sense, the whole nation of Israel is collectively mentioned as God's son (Ex. 4:22, Hos. 11:1) or “the sons/children of the Lord your God” (Deut. 14:1). In the coronal sense, the OT uses the title son of God for the anointed king of Israel (2 Sam. 7:14, Ps. 2:7;82:6).
Angels are called sons of God because they are a direct creation of God, much like Adam was (Lk. 3:38), unlike all other human beings. The children of Israel are called so corporately because collectively they were to serve His purposes on this earth as a firstborn would serve his father (Ex. 4:22-23; Jer. 31:9). The Israelite king was called the son of God because he was to mirror God, the true king of Israel, in all his actions as he sat on the throne of God (1 Chr. 29:23). So in the case of the angels, the title is used by virtue of their creation; in the case of Israel, it was used by virtue of their collective redemption; and in the case of the kings, the title was used by virtue of their theocratic function.
However, only in the New Testament do we find the title used in an individual relational sense. It applies strictly to those who receive Christ and are regenerated by the Holy Spirit (Jn 1:12, Rom. 8:14). Through individual salvation, believers receive this legal right to be heirs of God and co-heirs of Christ's inheritance (Gal. 4:5-7). By the Spirit, we can thus call God, Abba, Father (Rom. 8:15). It also reflects a change of nature and character which will be visible through peacemaking and holiness (Matt. 5:9). Finally, the bodily resurrection of believers is called the manifestation of the sons of God (Rom. 8:19).
While Israel was also redeemed, albeit corporately, and is a shadow of the fullness of salvation we enjoy in the NT, not every Israelite could thereby claim he was a son of God individually. Moreover, in OT Israel, even those who were not truly regenerate could still claim to be among the sons of God, simply because they belonged to corporate Israel (Rom. 9:6). The nature of the church, however, is not so. Only those who are truly born again are part of the church (Heb. 8:11) and can claim to be sons of God.
So, in conclusion, while the term "sons of God" is used in both the Old and New Testaments, the sense in which they are used is quite different. For angels, it's a creational sense; for corporate Israel, it's a covenantal or national sense; for Israelite kings, it's a royal or theocratic functional sense. Only in the New Testament is it used in an individual, redemptive, relational sense.
Finally, when it comes to Jesus, He is called the Son of God, because in His case, the title is used in the ontological sense. Jesus is not a created being, but one who has the same substance as the Father, sharing in His divine nature and eternal essence. In this sense, the title is not used for anyone else in the entire Bible (Heb. 1:5).
