Romans 1:16b

I am not ashamed of the gospel, because it is the power of God for the salvation of everyone who believes: first for the Jew, then for the Gentile.

(v16, NIV)

Paul is not ashamed of the gospel. The gospel brings shaming, but he does not need to be ashamed of it. He tells us why he isn't ashamed of it. He's not ashamed of it because of its power. It's the power of God for salvation. This is the power for salvation of everyone who believes. How is this power? Why is it power? Well, it could refer to the power of salvation to create converts. It could also refer to the future results of salvation as it keeps converts. John Piper argues that it is the latter.

Paul is talking about not being ashamed. Would the fact that the gospel makes converts keep him from being ashamed? Look at all these other religions, do they not make converts? If Paul were referring to that, it would be a weak assurance to rely on the power of the gospel to make converts. Instead, it is the power of the gospel to keep converts and usher them into future saving glory with God the father. This goes along with the fact that Jesus looked towards the joy set before him. We as believers look towards Jesus and hope and trust in the future saving work of the gospel at the end. This is the power of God.

You might say that other religions have their method of "salvation". Sure, they do in a sense. However, they are largely based upon what you do and your works and your deeds. What is the power of this gospel we hope and trust in? This is solely upon God. We don't save ourselves. This is nothing of us and our might and our efforts. This saving power of the gospel is entirely upon God who is more than able to save us. We are weak and unable to redeem ourselves from God's righteous judgment. This is the power of the gospel, not our efforts but Christ's powerful saving work on the cross to redeem us. Other religions do not have this. Instead, it is man-made and a trust in self and own deeds. Believe, trust and rely upon God and his power in his gospel to save.

People who are saved continue in their belief. If you "fall away", then you never were saved to begin with. God keeps those whom he chooses to save. In a sense, you can run all you want, but you can't escape it. So Paul wishes to preach the gospel to these believers. Not only does the gospel convert people, but it saves those who believe. He wishes to encourage them and remind them and grow them in their salvation. This too is the power of God. This isn't a onetime salvation but a continual process that continues in power through our lives. This is the power to save us and keep us till the end.