Albert Barnes Commentary Acts 12:9

"And he went out, and followed; and he knew not that it was true which was done by the angel, but thought he saw a vision." — Acts 12:9 (ASV)

And wist not: Did not know.

Saw a vision. That is, it was a representation made to his mind, similar to what he had seen before. Compare Acts 10:11-12. It was so astonishing, so unexpected, so wonderful that he could not realize that it was true.

The word "wist" means "knew." For the phrase "that it was," compare Psalm 126:1. The word "true" is used here in the sense of "real." For "vision," see also Acts 10:3, 17.