Why were the Jewish believers astonished that the Gentiles received the Holy Spirit?
The Jewish believers of the circumcision were astonished because this was the first instance of Gentiles receiving the Holy Spirit and being saved without adhering to the ritual of Judaism or becoming proselytes. The Spirit's falling on them demonstrated their faith apart from those Jewish ritual requirements.
Les teaches that there were six Jews who went with Peter, making seven Jews of the circumcision, and they were astonished. He points out that if Gentiles had been saved for the previous six, seven, or eight years, astonishment would make no sense; but they had not been. This was the first instance, and it was an astonishing fact that Gentiles could be saved without adhering to the ritual of Judaism, proselytizing, and all the rest. While Peter was still preaching, the Holy Spirit fell on all who heard the word, because they had become believers and believed what Peter was saying. The Spirit manifested their faith, and then Peter said they had to be baptized.
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.