Now to him who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages (ESV)
After all that Paul has written over the last previous chapters it would be easy to be intimidated. We are called to a holy standard of walking with Jesus Christ. How are we to accomplish this admirable task? Even Paul himself spoke of his struggle in Chapter 7 of not doing the things he wanted to do, while doing the very things that he knew he should not do.
The answer is found in the words of this wonderful verse. "To him who is able to strengthen you..." He is saying that the strength that we need to live a godly life is not found in us "trying harder." We are simply not strong enough to live a life that pleases God. It is God himself who is able to strengthen us. This promise is according to the Gospel and preaching of Jesus Christ, which Paul has faithfully delivered.
It is heart breaking to meet Christians who are constantly defeated in their walk with Christ. They recognize their own short-comings, but they do not know what to do about it. Many of them try to do more works. Others punish themselves by excessive fasting or other ascetic practices. What is the solution? They need to find their strength in the one who is able to strengthen them; God himself.
The mystery that Paul says has been revealed is this salvation that is for all people, not just the Jews. In our modern times we do not know why this is such a mystery. The Jews who would have read this letter to the church at Rome would have understood this mystery very well. The idea of Gentiles being recipients of the Gospel of Jesus Christ was shocking. How thankful we should be for the revealing of this mystery.
Are you struggling to live for Christ? Do you feel defeated daily? Are you trying to be righteous in your own efforts? Give up. Rest completely in God's grace. He alone is the one who is able to strengthen you.