And after the reading of the law and the prophets the rulers of the synagogue sent unto them, saying, [Ye] men [and] brethren, if ye have any word of exhortation for the people, say on.
Note 3 at Ac 13:15: The first five books of the Old Testament, written by Moses, were called "the law" or the "law of Moses" (Ac 28:23), while the rest of the Old Testament was called "the prophets" or "the prophets and the psalms" (Lu 24:44).
Note 4 at Ac 13:15: The Jewish synagogues had degenerated to the point that they had become a favorite hangout for devils (Mr 1:23 and Lu 4:33). Jesus, as well as Paul, delivered some of His most stinging rebukes in the synagogues (Mr 3:5; Lu 4:22-29; Ac 13:46, and 18:6), and Jesus spoke of the hypocrites in the synagogues (Mt 6:2).
A parallel can be drawn between the synagogues of Paul's day and the merely religious churches today. They both have a form of godliness but deny the power thereof, and Paul said to turn away from such (2Ti 3:5). Why then did Paul continually go into the synagogues of the Jews (see note 2 at Ac 13:14)?
The synagogues in Paul's day did not have a paid minister who preached but rather a man who was appointed ruler of the synagogue (see note 1 at Mt 9:18). He simply directed the synagogue meetings but did not preach at them. It was customary to read scriptures and then have people in the congregation comment on them. If those who were known for their insight into the things of God were present, they were asked to stand and speak, as Paul and Barnabas were here. Therefore, the synagogues provided them with an opportunity to minister to others, not to be ministered to.
Paul disputed boldly in the Ephesian synagogue for three months, but when the Jews rejected his message, he took his converts and separated them from the synagogue (Ac 19:8-9). Therefore, the scriptural precedent set by Paul is, "Associate with unbelievers when you have the opportunity to minister to them, but don't put yourself in a position where they are ministering to you."