Romans 15:17

The apostle here gives some account of himself and of his own affairs. Having mentioned his ministry and apostleship, he goes on further to magnify his office in the efficacy of it, and to mention to the glory of God the great success of his ministry and the wonderful things that God had done by him, for encouragement to the Christian church at Rome, that they were not alone in the profession of Christianity, but though, compared with the multitude of their idolatrous neighbours, they were but a little flock, yet, up and down the country, there were many that were their companions in the kingdom and patience of Jesus Christ. It was likewise a great confirmation of the truth of the Christian doctrine that it had such strange success, and was so far propagated by such weak and unlikely means, such multitudes captivated to the obedience of Christ by the foolishness of preaching. Therefore Paul gives them this account, which he makes the matter of his glorying; not vain glory, but holy gracious glorying, which appears by the limitations; it is through Jesus Christ. Thus does he centre all his glorying in Christ; he teaches us so to do, Co1 1:31. Not unto us, Psa 115:1. And it is in those things which pertain to God. The conversion of souls is one of those things that pertain to God, and therefore is the matter of Paul's glorying; not the things of the flesh.

— Matthew Henry's Concise Commentary (public domain)

I have therefore whereof I may glory--or (adding the article, as the reading seems to be), "I have my glorying." through--"in" Christ Jesus in those things which pertain to God--the things of the ministry committed to me of God.

— Jamieson-Fausset-Brown Commentary (public domain)