What does it mean that Christians are adopted by God?

from the old testament

• The Old Testament does not present adoption as a formal legal practice, but it reveals God's deep concern for the fatherless, calling His people to defend and care for them (Deut. 10:18; Ps. 68:5). Though examples like Moses (Ex. 2:10), Esther (Est. 2:7), and Mephibosheth (2 Sam. 9:7) aren’t legal adoptions, they reflect God’s heart to include the vulnerable in His family, pointing forward to the spiritual adoption fulfilled in Christ.

from the new testament

• Through adoption, God has permanently added those who trust in Him to His family of believers (John 1:12).
• The concept of adoption into God's family is another name for being "born again" like Jesus talked about with Nicodemus (John 3:3–6). In ancient Jewish society, adoption was not a normal part of culture, which is why Jesus used the more common Jewish idea of being born again when speaking with Nicodemus. In both the metaphor of being born again and of being adopted, we become children of God when we were not originally born into His family.
• Adoption grafts us into a new family line and gives us a new inheritance. When we are adopted into God's family, we become kingdom co-heirs with Christ, sharing in His glory and suffering, and supporting each other as the body of Christ (Romans 8:17; 1 Corinthians 12:25–26).
• God sent Jesus to earth "so that we might receive adoption as sons" (Galatians 4:5). The metaphor of adoption is used throughout the Bible to give a picture of how we become sons and daughters of God through the sacrifice of Jesus.
• Once we have been adopted into the family of God, He indwells us with the Holy Spirit who reminds us that we are God's children: "And because you are sons, God has sent the Spirit of his Son into our hearts, crying, 'Abba! Father!' So you are no longer a slave, but a son, and if a son, then an heir through God" (Galatians 4:6–7).
• When we place our faith in Jesus Christ, we are adopted into God's family. He cancels our debt to sin, gives us His name, and makes us heirs of His kingdom. He adopts us because of His great grace and love for us. Ephesians 2:4–6 describes this miracle: "But God, being rich in mercy, because of the great love with which he loved us, even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved—and raised us up with him and seated us with him in the heavenly places in Christ Jesus."

implications for today

It is important to note that while the language of the Bible often uses the word "sons" when talking about adoption into God's family, Paul was writing to both male and female believers. God’s gift of adoption is free to all who believe in Him, regardless of race, gender, or life circumstances: “But to all who did receive him, who believed in his name, he gave the right to become children of God, who were born, not of blood nor of the will of the flesh nor of the will of man, but of God” (John 1:12). We have an amazingly good God who is gracious enough to open His arms wide enough for all of His believing followers to become His children—and nothing can take us away from Him: “I give them eternal life, and they will never perish, and no one will snatch them out of my hand” John 10:18.