Acts 15:22-23, 28-29 (RSV) Then it seemed good to the apostles and the elders, with the whole church, to choose men from among them and send them to Antioch with Paul and Barnabas. They sent Judas called Barsab'bas, and Silas, leading men among the brethren,  with the following letter: . . .  “For it has seemed good to the Holy Spirit and to us to lay upon you no greater burden than these necessary things:  that you abstain from what has been sacrificed to idols and from blood and from what is strangled and from unchastity. If you keep yourselves from these, you will do well. Farewell.”
A prominent online anti-Catholic Calvinist polemicist wrote (paraphrasing) that the Jerusalem Council, which was attended by St. Peter, St. Paul, St. James, issued a letter only to a small group of Christians, and specifically to the Gentiles (in Antioch, Syria, and Cilicia, and was not intended to bind the entire Christian Church.
This particular letter (whether it was the only one, or one of several), was delivered at Antioch (15:30). Then we are informed that Paul and Silas “went through Syria and Cili’cia, strengthening the churches” (15:41). Thus, all three areas mentioned in 15:23 were indeed informed of this one decree, sent via letter with apostles.
But were those all the areas that Holy Scripture tells us were bound by the decrees of the council? The text that follows shows that this is not the case. Paul went next to “Derbe and to Lystra” (16:1). Derbe was in Asia Minor (current-day Turkey), and was then considered part of the region of Lycaonia. Granted, it wasn’t far from Cilicia, but it is a different locality. Lystra was a little north and west of Derbe.
Three verses later is a passage crucial to this discussion:
Acts 16:4-5 As they went on their way through the cities, they delivered to them for observance the decisions which had been reached by the apostles and elders who were at Jerusalem.  So the churches were strengthened in the faith, and they increased in numbers daily.
Note that this was after St. Paul had visited two cities that were not part of the three regions mentioned with regard to the specific letter mentioned in Acts 15:22-29. And the broad, sweeping language appears to imply that other local churches would be included also.
Moreover, the phrase, “the churches were strengthened in the faith” echoed the phrase, “strengthening the churches” (15:41), which was written in conjunction with the specific letter that we know about.
It is, therefore, quite reasonable to surmise either that 1) there were more letters of the sort that Acts 15:23 mentions, and/or 2) it was understood by St. Paul that he was to deliver the message of the decisions or decrees of the council, far and wide, in his travels.
The rest of chapter 16 details the many places Paul and Silas visited (western Turkey, island in the Aegean Sea, and Greece), where (presumably), they “delivered to them for observance the decisions which had been reached . . . at Jerusalem.”
No one need merely take my word and accept my argument in this respect. Several Protestant commentaries (for Acts 16:4-5) – all available online — back me up:
Benson Commentary [T]hese decrees . . . belonged equally to all the Gentile converts everywhere.
Barnes’ Notes on the Bible The word translated “decrees” occurs in Luke 2:1, “A decree from Caesar Augustus”; in Acts 17:7 “The decrees of Caesar”; in Ephesians 2:15; and in Colossians 2:14. It properly means a law or edict of a king or legislature. In this instance it was the decision of the council in a case submitted to it, and implied an obligation on the Christians to submit to that decision, since they had submitted the matter to them. The same principles, also, would be applicable everywhere, and the decision, therefore, at Jerusalem became conclusive.
Gill’s Exposition of the Entire Bible And as they went through the cities,…. Of Derbe, Lystra, and Iconium, and others in Lycaonia, and in Phrygia and Galatia; the Arabic version reads, “they both”; that is, Paul and Barnabas: they delivered them the decrees for to keep; they gave the churches, in these cities, the sentiments, and determinations to be observed and followed by them: that were ordained of the apostles which were at Jerusalem . . .
Matthew Henry Commentary He went through the cities where he had preached the word of the Lord, . . . All the churches were concerned in that decree, . . .
Robertson’s Word Pictures in the New Testament
They delivered them (παρεδιδοσαν αυτοις — paredidosan autois). Imperfect active, kept on delivering to them in city after city. This is a proof of Paul‘s loyalty to the Jerusalem compact (Knowling). . . .
The decrees (τα δογματα — ta dogmata). Old word from δοκεω — dokeō to give an opinion. It is used of public decrees of rulers (Luke 2:1; Acts 17:7), of the requirements of the Mosaic law (Colossians 2:14), and here of the regulations or conclusions of the Jerusalem Conference. Silas was with Paul and his presence gave added dignity to the passing out of the decrees, a charter of Gentile freedom, since he was one of the committee from Jerusalem to Antioch (Acts 15:22, Acts 15:27, Acts 15:32).
Which had been ordained (τα κεκριμενα — ta kekrimena). Perfect passive articular participle of κρινω — krinō to judge, emphasizing the permanence of the conclusions reached by the apostles and elders in Jerusalem.
For to keep (πυλασσειν — phulassein). This present active infinitive likewise accents that it is a charter of liberty for continual living, not a temporary compromise.
Expository Notes with Practical Observations on the New Testament To whom these decrees were delivered; namely, to the churches, as they passed along through the several cities; so many cities, so many churches: the whole company of Christians within a city and the adjacent territory, . . .