John Calvin Commentary Acts 6:15

"And all that sat in the council, fastening their eyes on him, saw his face as it had been the face of an angel." — Acts 6:15 (ASV)

And when they had beheld. In courts of judgment, people commonly turn their eyes toward the accused when they expect him to make his defense.

He says that Stephen appeared like an angel; this does not refer to his natural face, but rather to his countenance at that moment.

For while the countenance of those who are accused is usually pale, and they stammer in their speech and show other signs of fear, Luke teaches that there was no such thing in Stephen. Instead, a certain majesty appeared in him.

For Scripture sometimes uses a likeness to angels in this sense, as in 1 Samuel 24:9, 2 Samuel 14:17, and 2 Samuel 19:27.