For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, “Abba, Father.”
~Romans 8:15 (NKJV)

When we read about adoption in the New Testament, we should not think that it is simply referring to what we call adoption today (providing parents to a child with no mother or father). The son of a Roman man would be adopted, or set in place, as a full heir by his own natural father. The boy moved from being a mere child to being a full son–or from being merely considered a created thing to being considered the true offspring of the father. It is the same when we become born again Christians–we move from being merely created things to being sons of God. We are taken out of the dead “Race of Adam”, and placed into the living “Race of Christ” which makes us like Christ: sons of God, and full heirs to His kingdom.

And of course the word “sons” is gender neutral. It is used simply for this example, and it refers to both women and men–just like the “Bride of Christ” refers to the women and men who make up the Church.

Spirit of Adoption
(πνεῦμα υἱοθεσίας~original Greek word used in Romans 8:15 translated to adoption)
The Spirit of God, producing the condition of adoption.

Ὑιοθεσία adoption, is from υἱός son, and θέσις a setting or placing: the placing one in the position of a son. Mr. Merivale, illustrating Paul’s acquaintance with Roman law, says:

“The process of legal adoption by which the chosen heir became entitled not only to the reversion of the property but to the civil status, to the burdens as well as the rights of the adopter – became, as it were, his other self, one with him… this too is a Roman principle, peculiar at this time to the Romans, unknown, I believe, to the Greeks, unknown, to all appearance, to the Jews, as it certainly is not found in the legislation of Moses, nor mentioned anywhere as a usage among the children of the covenant. We have but a faint conception of the force with which such an illustration would speak to one familiar with the Roman practice; how it would serve to impress upon him the assurance that the adopted son of God becomes, in a peculiar and intimate sense, one with the heavenly Father”.

(Vincent Word Studies~quoting from “The Conversion of the Roman Empire” by Charles Merivale)