John Calvin on Acts 15:18

18. Known from the beginning. This is a prevention, “Prolepsis,” an anticipation to put away the hatred which might have risen upon the novelty; for the sudden change might have been suspected, and therefore did it trouble weak minds. Therefore James preventeth, showing that this was no new thing with God, though it fell out suddenly otherwise than men thought; because God saw, before the world was created, what he would do, and the calling of the Gentiles was hidden in his secret counsel. Whereupon it followeth, that it must not be esteemed according to the sense of man. Furthermore, James hath respect unto the words of the prophet, when he affirmeth that God, who should do all these things, was also the author of the prophecy. Therefore, his meaning is, that, seeing God speaketh by his prophet, he saw then, yea, from the very beginning, “Ab ultima aeternitate,” from the remotest eternity that neither uncircumcision nor anything else should let him, but that he would choose the Gentiles into his family. Nevertheless, there is comprehended under this a general exhortation, that men do not take upon them to measure, with the small measure of their wit, the works of God, the reason whereof is oftentimes known to none but to himself; but rather let them cry, being astonished, “Exclament cum stupore,” exclaim in amazement. that his ways are past finding out, and that his judgments are too deep a depth, (Romans 11:33.)

Acts 15:19-21

19. Wherefore, I think that we ought not to trouble those who of the Gentiles are turned to God: 20. But that we must write unto them, that they abstain from the filthiness of images, and from fornication, and from strangled, and from blood. 21. For Moses of old time hath those in every city which preach him, when he is read in the synagogues every Sabbath day.