Theologically, adoption is the act of God by which believers become members of "God’s family" with all the privileges and duties of family membership. The term "Sons of God" includes both men and women who are considered God’s children (Isaiah 43:6; 2 Corinthians 6:18).
According to the New Testament, everyone is sinful by nature, and so are "children of wrath" (Ephesians 2:3). However, those whom God loves become "children of God" by grace (1 John 3:1). This adoption comes from God's love. It is based on Jesus Christ, who is uniquely the Son of God. The term "Son of God" refers mainly to Christ’s divine nature (Matthew 11:25–27; 16:16–17). Jesus shares the same substance and glory as the Father. In the Trinity (the three persons of God), Christ is known as the second person. He is different from the Father because he is "the only begotten Son." This means he is the only one of his kind. Believers in Christ, though "adopted," are not equal to the uncreated, divine Son.
Even so, in Jesus, sinners are loved and chosen by God the Father to become his children by adoption (Ephesians 1:4–6). This adoption is made possible by Christ the Redeemer. Through his death and resurrection, he destroyed sin and its penalty, restoring the righteousness and life needed for sonship.
Christ is the head of the "new covenant" as the one who made it possible and the one who pays their debts. His followers become God’s heirs and Christ's joint heirs (Romans 8:17). God gives them the Holy Spirit, the Spirit of his Son, as the Spirit of adoption (Romans 8:15; Galatians 4:6). The Spirit in them assures believers that they are indeed God’s children and enables them to call God "Father" (Romans 8:15–16). This closeness with the Creator and Savior in prayer is one benefit of adoption.
Adoption was a benefit given to God’s people under the "old covenant" (Romans 9:4). Both Israel as a whole and individual Israelites knew God as Father (Isaiah 64:8–9; Hosea 11:1). Since the New Testament sees adoption as only possible through Jesus Christ, Israel's adoption before Jesus's coming made them equal to servants (Galatians 4:1–7). In Jesus, the benefit of being a child was extended to include both Jews and gentiles (Galatians 3:25–29). Although adoption is a benefit enjoyed in the present experience of God's people (1 John 3:1), its full extent is realized only at their resurrection from the dead (Romans 8:21–23).