Acts 15:21

For Moses of old time hath in every city them that preach him,
&c.] That is, for many years past, even from the times of Ezra, the law of Moses has been publicly expounded by them, whom the Jews call Derashim, preachers, or expounders, in every city where there was a synagogue; and every city belonging to the Jews, were obliged to build a synagogue, yea, they were obliged to do it where there were but ten Israelites ^F14: this is given by James as a reason why the Gentiles should be wrote unto concerning the above things; because that they hearing the law read and expounded every week, would be ready to conclude that they were obliged to submit unto it, as to circumcision, and other things; unless they were told that they were free from it; only in order to maintain peace with their brethren the Jews, it would be necessary for them to abstain from the above things: and it may also carry in it a reason, why the Jews need not be wrote unto, and why they had no reason to complain for thus writing to the Gentiles; since they had the law read and explained to them every week, and there would be no attempt to make any alteration in that form of service:

being read in the synagogues every sabbath day;
(See Gill on Acts 13:15).