Our obligation (v12-14) - Christians are no longer “obliged” to render obedience to their sinful nature, even though at times we can feel it’s influence. In fact, Paul strongly warns his readers that they will be damned if they continue to live according to their sinful nature! This warning must be balanced with Romans 8 teaching that truly regenerate believer, while often committing sin, will be infallibly prevented from living a sinful lifestyle by the Spirit within. The Christian is to put to death the misdeed of the body; we are responsible to kill off our sinful deeds. Holy Living is achieved by our constantly living out the life placed within us by the Holy Spirit. Our ‘self mortification’ is necessary but never apart from, or distinct to, the activity of the Holy Spirit within us.
Our adoption (v15- 16) - Paul argues that one of the roles of the Holy Spirit is that He makes the objective fact of our adoption a subjective reality in our experience (Eph1v4). There can be a tendency in those who don’t know the Spirit in his fullness to live as slaves rather than as sons. We have received the Spirit of God’s Son, and He evokes in us the cry of ‘Abba, Father’ (Gal 4:6). When we cry out by the Spirit ‘Abba Father’ our relationship to God the Father is like (but not exactly like) Jesus’ relationship to God the Father. We are God’s children, the Holy Spirit works in us so we have inward assurance of God’s love and His fatherhood.
His heirs (v17) Adopted children are heirs of a future inheritance, a future blessing. Christians are “heirs of God” not meaning that we inherit God himself, but that we inherit what God has promised to his people. Because we are one with Christ, we are his fellow heirs, assured of being “glorified with him”. But at the same time, this “oneness” means we will follow Christ’s own road to glory “suffering with him” as Philippians 1v29 reminds us. Suffering for being a Christian is, according to Paul, the condition of our receiving our inheritance;
In Conclusion - The Holy Spirit dwelling within obliges us to live His way (v12) and He enables us to put to death the misdeeds of our body (v13). He leads us as God’s children (v14) and bears witness to our spirit that we are what we are (v16). He himself is also the foretaste of the inheritance in glory (v17). It is His indwelling which makes a fundamental difference between our old way of life and living out Romans 8.