John Calvin Commentary Acts 1:9

"And when he had said these things, as they were looking, he was taken up; and a cloud received him out of their sight." — Acts 1:9 (ASV)

The readers may learn from our Institutions what profit we reap from the ascension of Christ. Nevertheless, because it is one of the chief points of our faith, Luke therefore endeavors more diligently to prove this. Indeed, rather, the Lord Himself intended to put this beyond all doubt, since He ascended so visibly and confirmed its certainty by other circumstances.

For, if He had vanished secretly, then the disciples might have doubted what had become of Him. But now, since they, being in such an open place, saw Him taken up—Him with whom they had been speaking, whom they had also just heard speak, whom they beheld with their eyes, and whom they also saw being taken from their sight by a cloud—they had no reason to doubt where He had gone.

Furthermore, the angels are also there to bear witness to this. And it was necessary that the account should have been recorded so diligently for our sake, so that we may know assuredly that although the Son of God appears nowhere on earth, yet He lives in the heavens.

And this seems to be the reason why the cloud overshadowed Him before He entered into His celestial glory: so that His disciples, being content with their measure, might cease to inquire any further. And we are taught by this that our mind is not able to ascend so high as to take a full view of the glory of Christ. Therefore, let this cloud be a means to restrain our boldness, just as the smoke was, which was continually before the door of the tabernacle in the time of the Law.