Romans 15:20-22

Yea, so have I strived to preach Greek, ουτω δε φιλοτιμουμενον ευαγγελιζεσθαι, literally, being ambitious; or, it being the object of my ambition; namely, so far as Providence would permit me to indulge it; to preach the gospel, not where Christ was named Had been preached before by others: that is, This way I took, as to my choice of places where to preach, lest I should build on another man's foundation, and so decline the difficulties which attend the settlement of new churches, or should assume to myself the credit due to others. He generally, though not altogether, declined preaching where others had preached, having a holy ambition to make the first proclamation of the gospel in places where it was quite unheard of, in spite of all the difficulty and danger that attended the doing of it. And the providence of God seemed, in a special manner, to prevent his preaching where others had preached, (though not entirely,) lest his enemies, who sought every occasion to set light by him, should have had room to say that he was behind other apostles, not being sufficient for planting churches himself, but only for preaching where others had prepared his way; or that he declined the more difficult part of the ministry. But as it is written

According to that prophecy which is now fulfilling in my ministry; to whom he was not spoken of Namely, the Gentiles; they shall see See on Isaiah 52:15. And they that have not heard In former times; shall understand And obey the gospel. For which cause That I might not build on another man's foundation; I have been much hindered from coming to you Among whom Christ had been named. Or he means, that he had been hindered by the important work of planting the gospel elsewhere.