Why were the Jews who came with Peter astonished in Cornelius's house?
Because Gentiles in Cornelius’s house heard the salvation message and believed it, the Jewish believers with Peter were astonished. The Holy Spirit fell on them, and they spoke in other languages, proving God was doing something totally different from anything He had done before.
Les explains that “they of the circumcision” means the Jews who believed, men who like Peter had recognized that Jesus was the Christ. He notes that the six men who came with Peter, making seven with him, were utterly astonished that these Gentiles were hearing a salvation message and believing it. Peter was in the Gentile house of Cornelius, a Roman officer, proclaiming the Gospel of the Kingdom, and while he spoke the Holy Spirit fell on all who heard the word. Since this was something totally different than had ever been done before, God proved it to those seven Jews by the Gentiles speaking other languages.
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.