John Chrysostom
Behold the divine economy of God. Peter had not yet finished his speech, and the baptism of Cornelius and those with him had not yet been performed at his command. But since they showed a wondrous disposition of soul, received the beginning of the teaching, and believed that forgiveness of sins is surely given in baptism, the Spirit came down upon them. God does this in order to give Peter a strong defense. They not only receive the Spirit, but also begin to speak in tongues, which astonished those who came with Peter. Why is the matter arranged in this way? For the sake of the Jews, because they looked upon this with great hostility. Thus everywhere everything is done by God.
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Peter is almost only present here, being instructed that they, the apostles, must now turn to the Gentiles, and that this must come about through them. Do not marvel. If, after such events, there was indignation both in Caesarea and in Jerusalem, what would there have been if these events had not occurred? For this reason they take place in an extraordinary way. [...] The writer did not say that Peter was amazed, but “those of the circumcision.” He knew what was happening. Indeed, one had to marvel that they too believed. They were not indignant when they heard that they had believed. Rather, they were indignant when they heard that God had given them the Spirit, when Peter explained his vision and said, “God has shown me that I should not call any man common or unclean” (Acts 10:28). Thus he already knew this beforehand. Therefore he prepares a speech about the Gentiles, in which he shows that they were no longer Gentiles once faith had appeared in them.