For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
Note 31 at Ro 8:15: This "spirit of bondage" is a reference to the old sin nature (see note 9 at Ro 5:21). The fact that Paul said, "Ye have not received the spirit of bondage again" (emphasis mine), is a further testimony that our old sin nature is not just ceremonially dead but that it is totally gone from our lives (see note 3 at Ro 6:2).
Note 32 at Ro 8:15: We are sons of God by adoption. Jesus was the Son of God by nature. As Jesus said to the Jews, we were of our father, the devil (Joh 8:44 and Eph 2:3), but Jesus purchased us and made us adopted sons of God.