Adoption is the legal process by which an individual receives another into his/her family. The adopter accepts parental responsibility for the adoptee and the adoptee simultaneously receives the name, privileges and responsibilities of the new family.
There is not a great deal about adoption in the Old Testament. This is probably because other rules and regulations were in place and delineated in scripture to aid both orphans and the childless.
The New Testament, on the other hand, addresses the matter of adoption specifically. New Testament adoption can be defined as an act of God's grace, through which He relationally accepts individuals into His family with full blessings and privilege.
Adoption was implicit in Jesus' teaching. He referred to God as Father (Matt. 5:16; 6:9; Luke 12:32). In John 1:12 He said that those who receive Him become the "children of God." The Bible further states that Jesus' mission is to bring "many sons to glory" and that He is not ashamed to call those who believe in Him "brothers" (Hebrews 2:10, 11).
However, a more comprehensive description of spiritual adoption is found in apostolic writings. Paul writes that believers "...did not receive the spirit of bondage again to fear, but...the Spirit of adoption by whom we cry out, "Abba (daddy), Father" (Romans 8:15). In verse 17 he adds, "...if children, then heirs—heirs of God and joint heirs with Christ..."
John, the beloved disciple adds, "Behold what manner of love the Father has bestowed on us, that we should be called children of God!"(1 John 3:1). And Peter further clarifies that believers have received not only blessings, inheritance, nurture and names, but they have taken on the very nature of God! He writes that believers have "been given...exceedingly great and precious promises, that through these you may be partakers of the divine nature..." (2 Peter 1:4).
There were 2 grade 1 boys claiming to be brothers, but their teacher noticed that their birthdays were 4 months apart. When she asked them how this could be, they said they would ask their dad that night. They did and he replied that the reason was that one of them was adopted and it didn't matter which one.
However, they wanted to know which of them was adopted so they pressed the father on the matter. He told them, "I'm afraid I cannot help you. You see", he explained, "I have loved and accepted you both so equally, that I cannot remember which one of you is adopted." I believe this story captures the personal and powerful nature of spiritual adoption.
As adopted children, "God has sent forth the Spirit of His Son into your hearts, crying out, "Abba (Daddy), Father!" (Galatians 4:6).