Romans 15:15

But I have written to you the more boldly in part, as putting you in remembrance, because of the grace given me by God.

Paul says he has been given authority by the grace of God, to embolden him to write to all the Gentiles, exhorting and confirming their calling in Christ, so that he might show his concern in the service of the gospel as a teacher of …

These things therefore being manifest to us, and since we look into the depths of the divine knowledge, it behooves us to do all things in [their proper] order, which the Lord has commanded us to perform at stated times. He has enjoined offerings [to be presented] and service to be performed [to Him], and that not thoughtlessly or irregularly, but at the appointed times and hours. Where and by whom He desires these things to be done, He Himself has fixed by His own supreme will, in order that all things, being piously done according to His good pleasure, may be acceptable unto Him. Those, therefore, who present their offerings at the appointed times, are accepted and blessed; for inasmuch as they follow the laws of the Lord, they sin not. For his own peculiar services are assigned to the high priest, and their own proper place is prescribed to the priests, and their own special ministrations devolve on the Levites. The layman is bound by the laws that pertain to laymen.

"Nevertheless, I have written the more boldly unto you in some sort." Observe the lowly-mindedness of Paul, observe his wisdom, how he gave a deep cut in the former part, and then when he had succeeded in what he wished, how he uses much kindliness next. For even without what he has said, this very confession of his having been bold were enough to unstring their vehemency. And this he does in writing to the Hebrews also, speaking as follows, "But, beloved, I am persuaded better things of you, and things which belong unto salvation, though we thus speak." And to the Corinthians again, "Now I praise you, brethren, that ye remember me in all things, and keep the ordinances, as I delivered them to you." And in writing to the Galatians he says, "I have confidence in you, that ye will be none otherwise minded." And in all parts of his Epistles one may find this to be frequently observed. But here even in a greater degree. For they were in a higher rank, and there was need to bring down their fastidious spirit, not by astringents only, but by laxatives also. For he does this in different ways. Wherefore he says in this place too, "I have written the more boldly unto you," and with this even he is not satisfied, but has added, "in some sort," that is, gently; and even here he does not pause, but what does he say? "As putting you in mind." And he does not say as teaching, nor simply putting in mind, but he uses a word which means putting you in mind in a quiet way. Observe the end falling in with the introduction. […]

Paul is telling them that he has not received anything new or wonderful which he is writing to teach them. Rather, he is just reminding them of things they have already learned.