I am not ashamed
of the gospel
This particular passage provides a brilliant illustration of Paul's ability to KNOW the scriptures, most likely he had memorized the entire Old Testament. Therefore he was able to quote from it at will as he often did while writing from prison without access to a written copy.
He refers to Habakkuk 2:1-4
"I will stand at my guard post.
I will station myself on the wall.
I will watch to see what he will say to me,
and what answer I will get to my complaint.
Then the Lord answered me,
'Write the vision.
Make it clear on tablets
so that anyone can read it quickly.
The vision will still happen at the appointed time.
It hurries toward its goal.
I won't be a lie.
If it is delayed, wait for it.
I will certainly happen.
It won't be late.
Look at the proud person.
He is not right in himself.
But the righteous person
will live because of his faithfulness."
Simply put, the gospel shows us both how righteous God is in his plan for us to be saved, and also how we may be made fit for eternal life. From first to last, God declares us to be righteous because of faith and faith alone!