GOD'S WORD® Translation
Peter followed the angel out of the cell. He didn't realize that what the angel was doing was actually happening. He thought he was seeing a vision.

American Standard Version
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.

Berean Study Bible
So Peter followed him out, but he was unaware that what the angel was doing was real. He thought he was only seeing a vision.

Douay-Rheims Bible
And going out, he followed him, and he knew not that it was true which was done by the angel: but thought he saw a vision.

English Revised Version
And he went out, and followed; and he wist not that it was true which was done by the angel, but thought he saw a vision.

King James Bible
And he went out, and followed him; and wist not that it was true which was done by the angel; but thought he saw a vision.

World English Bible
And he went out and followed him. He didn't know that what was being done by the angel was real, but thought he saw a vision.

Young's Literal Translation
and having gone forth, he was following him, and he knew not that it is true that which is done through the messenger, and was thinking he saw a vision,

Acts 12:9 Additional Translations ...