Why were the Jewish believers with Peter astonished at Cornelius's house?
The Jewish believers accompanying Peter were astonished because they witnessed the Holy Spirit being poured out on Gentiles, something unprecedented. Until then salvation had been exclusively Jewish, so Gentile conversion was unexpected. Acts 10:45 records their astonishment at this evidence of Gentile salvation.
Les teaches that Peter took six Jews with him to the house of Cornelius, making seven including Peter, because God's perfect number is seven. When those seven Jews, including Peter, saw the evidence of Gentiles being saved, they were astonished. This had never happened before, since salvation had been entirely Jewish up to that point. People wrongly think Gentiles were saved at John the Baptist or Pentecost, but Les says no, it was all Jewish. For a Gentile to be saved was an astonishment. Acts 10:45 says that they of the circumcision who believed, those who came with Peter, were astonished because on the Gentiles was poured out the gift of the Holy Ghost. Later in Acts 11, Peter and the six men returned to Jerusalem, and Jewish believers contended with him, asking why he went to uncircumcised men and ate with them. That reaction shows Gentiles had not been saved all along.
And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
More Questions
How many Jews went with Peter to Cornelius, and who were the ones astonished?
According to Acts 11, six Jews went with Peter, so the seven Jews of the circumcision who believed were astonished at what happened.
What happened to the circumcised Jewish believers with Peter in Acts 10:45 when the Holy Spirit fell on the Gentiles?
The six circumcised Jewish believers who came with Peter were astonished when they saw the Holy Spirit falling on the Gentile listeners, even though they already believed that Jesus is the Christ.