To preach the gospel in the regions beyond you. — It is clear, from Romans 15:19-24, that he is thinking (1) of Western Greece, (2) of Rome, (3, and chiefly) of Spain. There, apparently, he could hope to preach the gospel without even the risk of its being said that he was building on another man’s foundation.
And not to boast in another man’s line... — The words, like those of 2 Corinthians 10:15, are at once an answer to a charge and a tu quoque retort. “Spain! Illyricum!” he seems to say within himself. “Will you say that I am transgressing boundaries and working on another man’s lines there? Can you say that you are free from that charge in your work at Corinth?”