Acts 7:42
Then God turned, and gave them up to worship the host of heaven; as it is written in the book of the prophets, O ye house of Israel, have ye offered to me slain beasts and sacrifices by the space of forty years in the wilderness? Then God turned, and gave them up to worship the host of heaven; as it is written in the book of the prophets, O ye house of Israel, have ye offered to me slain beasts and sacrifices by the space of forty years in the wilderness?
When Israel turned from God in rebellion in the wilderness, God turned and gave them up to worship the host of heaven, the heavenly bodies. The appeal to the book of the prophets recalls the forty years in the wilderness and asks whether the slain beasts and sacrifices Israel offered there were truly offered to God. Israel's history in this shows that the law they trusted for salvation did not preserve them from idolatry.
The verse belongs to Stephen's review of Israel's history drawn from the Old Testament, beginning with Moses and the prophets, presenting its spiritual meaning made clear through Christ. In that testimony he made the prophecies relating to the Messiah clear, showing their agreement with the life and teachings of Christ as inspired proof of His mission, suffering, death, resurrection, and ascension, proving His identity as the Messiah. The testimony declares that it was the voice of Christ speaking through the prophets and patriarchs from the days of Adam onward, and that the ceremonies and symbols they described pointed to Him and are valueless if the Savior they reveal is rejected.
Saul remembered this part of Stephen's sermon after seeing Christ, and what he had accepted as blasphemy then appeared true, that the prophets had foretold Jesus' rejection, crucifixion, resurrection, and ascension, identifying Him as the promised Messiah.