Romans 8:15. Ye have received the Spirit of adoption, whereby we cry, Abba, Father.
Among the Greeks and Romans, when a man had no son he was permitted to adopt one even though not related. He might, if he chose, adopt one of his slaves as a son. The adopted son took the name of the father, and was in every respect regarded and treated as a son. Among the Romans there were two parts to the act of adoption: one a private arrangement between the parties, and the other a formal public declaration of the fact. It is thought by some that the former is referred to in this verse, and the latter in verse 23, where the apostle speaks of “waiting for the adoption.” The servant has been adopted privately, but he is “waiting” for a formal public declaration of the fact.
After adoption, the son, no longer a slave, had the privilege of addressing his former master by the title of “father.” This he had no right to do while a servant. See also Galatians 4:5-6.