John Calvin Commentary Acts 1:10

"And while they were looking stedfastly into heaven as he went, behold, two men stood by them in white apparel;" — Acts 1:10 (ASV)

Two men: He calls them this because of their form. For although they might indeed have had human bodies—a point I will not strongly argue for either way—it is certain they were not actually men. However, because this metonymia is commonly used in the Scriptures, especially in the First Book of Moses, I will not dwell further on this point.

Their white garments were a sign of rare and excellent dignity. For God intended by this, as by a clear sign, to distinguish them from ordinary people, so that the disciples would listen more attentively to them. Furthermore, He intended that we also, to this day, may know that this vision was shown to them by God.