8:12-17 So then, brothers, a duty is laid upon us--and that duty is not to our own sinful human nature, to live according to the principles of that same nature; for, if you live according to the principles of sinful human nature, you are on the way to death; but if by the spirit you kill the deeds of the body, you will live. For all who are guided by the Spirit of God, these, and only these, are the children of God. For you did not receive a state whose dominating condition is slavery so that you might relapse into fear; but you received a state whose dominating characteristic is adoption, in which we cry, "Abba! Father!" The Spirit itself bears witness with our spirit that we are children of God. If we are children then we are also heirs; and if we are the heirs of God then we are joint-heirs with Christ. If we suffer with him we shall also be glorified with him.
Paul is introducing us to another of the great metaphors in which he describes the new relationship of the Christian to God. He speaks of the Christian being adopted into the family of God. It is only when we understand how serious and complicated a step Roman adoption was that we really under stand the depth of meaning in this passage.
Roman adoption was always rendered more serious and more difficult by the Roman patria potestas. This was the father's power over his family; it was the power of absolute disposal and control, and in the early days was actually the power of life and death. In regard to his father, a Roman son never came of age. No matter how old he was, he was still under the patria potestas, in the absolute possession and under the absolute control, of his father. Obviously this made adoption into another family a very difficult and serious step. In adoption a person had to pass from one patria potestas to another.
There were two steps. The first was known as mancipatio, and was carried out by a symbolic sale, in which copper and scales were symbolically used. Three times the symbolism of sale was carried out. Twice the father symbolically sold his son, and twice he bought him back; but the third time he did not buy him back and thus the patria potestas was held to be broken. There followed a ceremony called vindicatio. The adopting father went to the praetor, one of the Roman magistrates, and presented a legal case for the transference of the person to be adopted into his patria potestas. When all this was completed, the adoption was complete. Clearly this was a serious and an impressive step.
But it is the consequences of adoption which are most significant for the picture that is in Paul's mind. There were four main ones. (i) The adopted person lost all rights in his old family and gained all the rights of a legitimate son in his new family. In the most binding legal way, he got a new father. (ii) It followed that he became heir to his new father's estate. Even if other sons were afterwards born, it did not affect his rights. He was inalienably co-heir with them. (iii) In law, the old life of the adopted person was completely wiped out; for instance, all debts were cancelled. He was regarded as a new person entering into a new life with which the past had nothing to do. (iv) In the eyes of the law he was absolutely the son of his new father. Roman history provides an outstanding case of how completely this was held to be true. The Emperor Claudius adopted Nero in order that he might succeed him on the throne; they were not in any sense blood relations. Claudius already had a daughter, Octavia. To cement the alliance Nero wished to marry her. Nero and Octavia were in no sense blood relations; yet, in the eyes of the law, they were brother and sister; and before they could marry, the Roman senate had to pass special legislation.
That is what Paul is thinking of. He uses still another picture from Roman adoption. He says that God's spirit witnesses with our spirit that we really are his children. The adoption ceremony was carried out in the presence of seven witnesses. Now, suppose the adopting father died and there was some dispute about the right of the adopted son to inherit, one or more of the seven witnesses stepped forward and swore that the adoption was genuine. Thus the right of the adopted person was guaranteed and he entered into his inheritance. So, Paul is saying, it is the Holy Spirit himself who is the witness to our adoption into the family of God.
We see then that every step of Roman adoption was meaningful in the mind of Paul when he transferred the picture to our adoption into the family of God. Once we were in the absolute control of our own sinful human nature; but God, in his mercy, has brought us into his absolute possession. The old life has no more rights over us; God has an absolute right. The past is cancelled and its debts are wiped out; we begin a new life with God and become heirs of all his riches. If that is so, we become joint-heirs with Jesus Christ, God's own Son. That which Christ inherits, we also inherit. If Christ had to suffer, we also inherit that suffering; but if Christ was raised to life and glory, we also inherit that life and glory.
It was Paul's picture that when a man became a Christian he entered into the very family of God. He did nothing to deserve it; God, the great Father, in his amazing love and mercy, has taken the lost, helpless, poverty-stricken, debt-laden sinner and adopted him into his own family, so that the debts are cancelled and the glory inherited.