Why does Paul claim to have fulfilled the ministry of the gospel from Jerusalem all the way around Illyricum?

When Paul dares to boast of his work of another, Christ, he can point to: 1. the extent of the area of his work; 2. the novelty of the result of the work; and 3. the realization of an old plan of salvation: [S]o that from Jerusalem and all the way around to Illyricum I have fulfilled the ministry of the gospel of Christ; and thus I make it my ambition to preach the gospel, not where Christ has already been named, lest I build on someone else’s foundation, but as it is written, Those who have never been told of him will see, and those who have never heard will understand (Romans 15:19–21). Almost immediately after his call at Damascus, Paul began to preach in Jerusalem and its surroundings (Acts 26:20). He had also appeared briefly in the synagogues in Damascus, but the real beginning of his work in the gospel was in Jerusalem. From the book of Acts we can deduce that once Paul entered Macedonia, he set course for Rome. His plans to travel to Rome failed time and again, however, just like many of his other travel plans. Paul almost never reached his goals directly. Often, he had to take outflanking movements, deviate from his planned route to evade enemies, or swerve from them to take on more urgent tasks by order of the Spirit. That is why in Romans 15:19 he writes that kukloi (turning around in a circle), he ultimately reached the western boundary of Greece. Paul did eventually reach Illyricum, present day Albania and surroundings, or to its boundary (it is unclear whether the Greek mechri means into or unto). We do not know when he was there (probably in the last period of the third missionary journey). The reference to Illyricum is not relevant in answering the question whether the apostle preached in this area. Rather, Paul mentions it to indicate how far he had come in the direction of Rome. He therefore does not mention Illyricum from the perspective of his missionary works, but from the perspective of Rome. For the Roman Christians, Jerusalem is a long distance away to the east. But Paul has meanwhile already reached the coast of the Adriatic Sea, from which you can cross over to Italy (for example from Apolonia in Illyricum to Brundisium in Italy) and then proceed to Rome. Looked at from the perspective of Rome, to travel to Jerusalem, you have to take the Via Appia (to the eastern city of Brundisium on the Adriatic coast, from where you cross the Adriatic Sea to Illyricum). From there you travel on the so-called Via Egnatia, which runs through Macedonia in an easterly direction. By mentioning Illyricum (the most westerly coast for crossing over to Italy and Rome), he indicates that he is just across the water. The entire territory, from Jerusalem almost to Italy has been Paul’s area of work. For the gospel this was still virgin territory, for all the other apostles worked in the direction of Egypt, Syria, and Babylon. The Lord guided Paul westward and he sought to cover more and more of the territory there. With his knowledge of Greek and with the years of his youth spent in Tarsus, he was better prepared for this territory. Still, this was not his most important motivation. He knew himself called to cultivate a new territory. He did not build on the foundation of others (as Apollos, Cephas, and others built on the foundation that Paul had laid in Corinth (1 Corinthians 3:6–10). In this way, Paul became a living manifestation of the realization of God’s plan of salvation during this time. The prophet Isaiah had already announced that the Gentiles, who lived far beyond the law and the revelation, would, in time, hear the good news about the suffering servant of the Lord (Isaiah 52:15). The Lord had taken Paul up into the same plan of salvation in which the readers in Rome had been taken up (Romans 15:7–12). Hence his frankness in writing to them.

The idea that in choosing his travel destinations mentioned here in Romans 15:19 Paul took into account the Old Testament passage of Isaiah 66:18–21¹ is rather speculative. The suggestion that a journey to the west by Paul could be connected indirectly to the promise of Isaiah 66:1–24, namely that the Gentiles will be gathered from all nations and tongues, is one made with hindsight, for in Romans 15:19 Paul is not making plans for further journeys, but is looking back on his work² Moreover, the text does not even contain a direct reference to Isaiah 66.^3,4