Romans 8:15-16 meaning

Paul now makes a clear distinction between the unconditional gift God gives us to be His child forever through simple faith, and the great reward He bestows on those who walk in obedience and dependence, even as Jesus walked.

So far, Paul has been prescribing the way to live contrary to the flesh, which is to live through the Spirit. He has used Spirit more times in this chapter than the prior seven chapters combined. Paul now turns his attention to a new aspect of faith; he's showing the next level of the newness of life we have in Christ. Paul has argued, practically, that it is better to walk in the Spirit rather than in the dead, sinful, former way of living. Now, at this point in his letter to the Roman Christians, Paul goes beyond his prior argument to talk about something even greater.

Speaking to the Roman Christians, Paul has already established that you have not received a spirit of slavery leading to fear again (v 15). We are no longer slaves to sin, and this new life we receive when we put our faith in Jesus's death and resurrection does not result in fear; it really is like being adopted into a family. So much so, that Paul declares but you have received a spirit of adoption as sons by which we cry out, "Abba! Father!" (v 15). 

God has taken us into His care, and we call Him Father. Paul expresses that when we believe on Jesus, the Spirit Himself testifies with our spirit that we are children of God (v 16). He gives us an inner testimony that we are His children. If you go to Israel today you can hear boys call their fathers "Abba," which simply means "Daddy."

This means that if we are hearing God's inner voice, it is a testimony to us that we are His. We can know we are His through faith in His word, simply having enough faith to look, hoping to be delivered from the poisonous venom of sin (John 3:14-15). But God graciously also gives us an inner voice; if that voice that testifies with our spirit then we are His children. Jesus promised we could be given new birth simply through believing. He then gives us the indwelling Holy Spirit to testify to us that we are His. Being place into Christ is something we gain and cannot lose. 

As Paul states in 2 Timothy, "If we are faithless, He remains faithful, for He cannot deny Himself." Since all believers are placed into Christ and His death, if God rejected us He would be rejecting Himself. We are God's children regardless of what we do. This is the teaching that offended Paul's opponents. They slandered Paul saying this teaching would lead to a conclusion that we ought to sin a lot (Romans 3:8). But Paul steadfastly asserts that becoming God's child by spiritual birth is like physical birth—it is just a gift. 

However, in the next section he will make clear that growing up is different—maturing and gaining the greatest benefit from life requires making good choices. All believers are children. Not all believers are sons.