John Calvin Commentary Acts 10:3

"He saw in a vision openly, as it were about the ninth hour of the day, an angel of God coming in unto him, and saying to him, Cornelius." — Acts 10:3 (ASV)

He saw in a vision. Luke presents 'vision' as a kind of oracle coming from God, so that we may know that Cornelius was brought to the faith of Christ in a heavenly manner. And because people are often deceived by trickery, Luke records the time to avoid all suspicion, when he mentions the ninth hour. At that time, they were accustomed to dividing the day into twelve hours, from sunrise to sunset. From this it follows that it was broad daylight when the angel appeared, so that the vision would be more evident.

Although seals were always added to visions to free God's servants from the fear of illusions, this assurance extended even to dreams. For when God appeared in dreams, such marks of certainty were nevertheless imparted to their minds that they would not be allowed to doubt.