John Calvin 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)

He knew not that it was true. He did not think that it was a vain or false illusion, as Satan often deludes people with trickeries; but true is understood here as that which is done naturally and in the ordinary human manner. For we must note the contrariety [antithesis] between the thing itself and the vision. Furthermore, though he thinks it is a vision, yet he willingly obeys. By this his obedience is proven: being content with the angel's command alone, he does not inquire or reason about what he must do, but does what he is commanded to do.