John Calvin Commentary Romans 15:19

"in the power of signs and wonders, in the power of the Holy Spirit; so that from Jerusalem, and round about even unto Illyricum, I have fully preached the gospel of Christ;" — Romans 15:19 (ASV)

So that from Jerusalem, etc. He also adds a testimony from the results, for the success that followed his preaching exceeded all human expectations. For who could have gathered so many churches for Christ without being aided by the power of God? From Jerusalem, he says, I have spread the gospel as far as Illyricum, not by rushing to my destination by a direct route, but by going all around and through the intervening countries.

But the verb πεπληρωκέναι, which, following others, I have translated as filled up or completed, means both to perfect and to supply what is lacking. Hence πλήρωμα in Greek means perfection as well as a supplement. I am inclined to explain it thus—that he spread the preaching of the gospel, as it were, by filling it up; for others had begun before him, but he spread it more widely.