Look carefully with me how “the obedience of faith” (at the end of verse 26) fits in these verses. Paul begins his doxology by saying that one of the reasons glory belongs to God is that he is able to strengthen us. Verse 25: “Now to him who is able to strengthen you . . . be glory forevermore.” Between that opening declaration that God can strengthen your faith and the closing acclamation that God is glorious, Paul unpacks the gospel as the very thing that God uses to strengthen our faith.
Follow the phrases he uses to unpack the gospel and you will see how he begins by saying the gospel strengthens our faith and ends by saying the gospel produces the obedience that comes from faith.
Verse 25 says he strengthens us “according to my gospel.” So he simply strikes the note that this message is “good news.” That is what gospel means. We are made strong by good news. Then he calls the gospel the preaching of Jesus Christ. That means that the heart of the gospel is good news about who Jesus Christ is and what he did when he came and died and rose again. He doesn’t tell us what that is because that is what the first eight chapters were about.