He thought he was seeing a vision.

Acts 12:6-11, And on the very night when Herod was about to bring him forward, Peter was sleeping between two soldiers, bound with two chains; and guards in front of the door were watching over the prison. And behold, an angel of the Lord suddenly appeared, and a light shone in the cell; and he struck Peter’s side and roused him, saying, “Get up quickly.” And his chains fell off his hands. And the angel said to him, “Gird yourself and put on your sandals.” And he did so. And he said to him, “Wrap your cloak around you and follow me.” And he went out and continued to follow, and he did not know that what was being done by the angel was real, but thought he was seeing a vision. And when they had passed the first and second guard, they came to the iron gate that leads into the city, which opened for them by itself; and they went out and went along one street; and immediately the angel departed from him. And when Peter came to himself, he said, “Now I know for sure that the Lord has sent forth His angel and rescued me from the hand of Herod and from all that the Jewish people were expecting.”

Peter had seen it all, walking away from his occupation of fishing to walk with Jesus, seeing the miracles of raising the dead, healing the sick, feeding the multitudes, seeing Jesus in all His divine glory on the mount of Transfiguration. He still did not believe what was happening when God sent His mighty angel to disarm the powers of darkness and set him free from prison! Ultimately, Peter would go to his death, crucified upside down refusing to recant His faith in the risen Lord Jesus Christ. How small is our faith?

Lord, increase our faith! Thank You that You move even when our faith is as tiny as a mustard seed!