John Calvin Commentary Acts 27:25

"Wherefore, sirs, be of good cheer: for I believe God, that it shall be even so as it hath been spoken unto me." — Acts 27:25 (ASV)

For I believe God. Paul tells them again from where he had such boldness, that he affirms that though they are amidst infinite gulfs of the sea, yet they shall all come safely to the harbor, specifically, because God had promised it would be so. In these words the nature of faith is expressed, where a mutual relationship is established between it and the Word of God, so that it may strengthen people's minds against the assaults of temptations.

And he does not only exhort the mariners by his own example to believe, but does, as it were, take upon himself the role of one who promises, so that he may win credibility for the divine message. What follows immediately concerning the island is a later sign, by which it may more plainly appear after the event that their voyage was not uncertain; otherwise, it would have been pointless for the mariners to know how they would escape.

Therefore, we see how God gives the safety He promised, providing a sign so that it may not seem to come by chance. Nevertheless, we must note that God still kept them in some doubt, partly so that He might exercise the faith of His servant, and partly so that they might all know that Paul learned that from the Holy Spirit, which he could not yet comprehend by human reason.

Nevertheless, Luke teaches in the text itself that Paul was not believed despite all this. For, since the mariners thought that some land began to appear to them, this did not align with the promise made concerning their arrival on an island. Therefore, we see how they were scarcely compelled, even by experience, to believe that he spoke the truth.