John Calvin Commentary Acts 10:45

"And they of the circumcision that believed were amazed, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Spirit." — Acts 10:45 (ASV)

The faithful were astonished. He calls them faithful who were still possessed by a wicked error. So the Lord does not immediately wipe away all clouds of ignorance from His people; yet these clouds do not obscure the value of their faith before God, because He pardons errors and graciously favors it, as if it were pure and faultless.

And yet it is a wonder that, though they knew Peter was sent by God, they were now amazed—as at something strange and new—that God gives the grace of His Spirit to those to whom He willed Christ now to be preached. But the sudden change is the cause of this amazement. For while God until that day had separated the Gentiles from His people as strangers and aliens, He now favors them both alike and lifts them up to the same degree of honor.

This example also teaches us how hard it is for us to extricate ourselves from errors once conceived, especially when they are long-standing.