Adam Clarke on Romans 15:15

Nevertheless - I have written - Not withstanding I have this conviction of your extensive knowledge in the things of God, I have made bold to write to you in some sort, απο μερους, to a party among you, as some learned men translate the words, who stand more in need of such instructions than the others; and I do this, because of the grace, δια την χαριν - because of the office which I have received from God, namely, to be the apostle of the Gentiles. This authority gave him full right to say, advise, or enjoin any thing which he judged to be of importance to their spiritual interests. This subject he pursues farther in the following verse.