Understatement...

Romans 1:16-17 For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes: first to the Jew, then to the Gentile. For in the gospel the righteousness of God is revealed—a righteousness that is by faith from first to last, just as it is written: “The righteous will live by faith.” 

Why is Paul so excited, so passionate?  Because he has discovered the Good News of salvation that transforms those who believe.  He had spent his life trying everything possible to have a right relationship with God, doing all He could, but he knew it was not enough.  Then he came to faith in Christ.  He realized Christ has done what needed to be done, and by faith in Him righteousness comes - our relationship with God can be restored, and our lives can be lived in hope, with purpose.  So, when Paul says he is "not ashamed", he is emphasizing the opposite: he wants to shout it out, to proclaim to the world what he has discovered - or rather, Who he has discovered: Christ, our Savior.