Adoption (the English word is derived from the Latin word meaning to choose - option is derived from the same word) is the acceptance of another person's child as one's own. Adoption is very ancient, and is commonly found everywhere in the world among many cultures. The Bible perspective on adoption expands the principle of natural adoption to include national and spiritual adoption. The last one, spiritual adoption, is "the point" of God's offer of eternal life, made available to everyone of all nations who will exercise their option to genuinely repent and truly obey Him.
Natural adoption was common during Bible History:
"And the child grew, and she brought him to Pharaoh's daughter, and he became her son; and she named him Moses, for she said, "Because I drew him out of the water." (Exodus 2:10 RSV)
"He had brought up Hadassah, that is Esther, the daughter of his uncle, for she had neither father nor mother; the maiden was beautiful and lovely, and when her father and her mother died, Mordecai adopted her as his own daughter." (Esther 2:7 RSV)
Physical Israel was adopted by God, not out of favoritism, but for a purpose:
"Thus says The Lord, Israel is My first-born son" (Exodus 4:22 RSV)
"For you are a people holy to The Lord your God; The Lord your God has chosen you to be a people for His own possession, out of all the peoples that are on the face of the earth" (Deuteronomy 7:6 RSV)
"They are Israelites, and to them belong the sonship, the glory, the covenants, the giving of the Law, the worship, and the promises; to them belong the patriarchs, and of their race, according to the flesh, is the Christ." (Romans 9:4-5 RSV)
Spiritual adoption is available to all who will repent and obey God:
"His divine power has granted to us all things that pertain to life and godliness, through the knowledge of Him who called us to His own glory and excellence, by which He has granted to us His precious and very great promises, that through these you may escape from the corruption that is in the world because of passion, and become partakers of the divine nature." (2 Peter 1:3-4 RSV)
"But to all who received Him, who believed in His Name, He gave power to become children of God" (John 1:12 RSV)
"For you did not receive the spirit of slavery to fall back into fear, but you have received the spirit of sonship. When we cry, "Abba! Father!" it is the Spirit Himself bearing witness with our spirit that we are children of God, and if children, then heirs, heirs of God and fellow heirs with Christ, provided we suffer with Him in order that we may also be glorified with Him" (Romans 8:15-17 RSV)
Fact Finder: What is meant by "children of God"?
See Children Of God