John Calvin Commentary Acts 15:18

"Saith the Lord, who maketh these things known from of old." — Acts 15:18 (ASV)

Known from the beginning. This is a preemptive point, intended to dispel the animosity that might have arisen from the novelty of the situation; for the sudden change could have been viewed with suspicion, and therefore it troubled those with weaker understanding. Consequently, James addresses this beforehand by showing that this was no new development for God, even though it occurred suddenly and differently from what people expected. This is because God saw, before the world was created, what He would do, and the calling of the Gentiles was hidden in His secret counsel.

From this it follows that it must not be judged by human understanding. Furthermore, James refers to the words of the prophet when he affirms that God, who would do all these things, was also the author of the prophecy. Therefore, his meaning is that, since God speaks through His prophet, He saw then, indeed, from the very beginning, that neither uncircumcision nor anything else would hinder Him, but that He would choose the Gentiles into His family.

Nevertheless, this also contains a general exhortation: that people should not presume to measure, with their limited understanding, the works of God, the reason for which is often known only to Him. Instead, let them exclaim in astonishment, that His ways are past finding out, and that His judgments are too deep a depth (Romans 11:33).