Why are forty years significant in Israel's history?
The forty years from the crucifixion to Rome's destruction of the temple marked a period of testing for Israel. During the book of Acts, God continued dealing with His covenant people, but at the end of that forty years Israel remained in unbelief, so God permitted Rome to destroy the city and temple and disperse the nation.
Les explains that the forty years from the crucifixion to the destruction of the temple by the Roman army are crucial. The book of Acts covers this forty-year period, during which God is still dealing with His covenant people, even though He begins going to the Gentiles before the forty years end. In Israel's history, forty years has always been a period of testing. At the end of that period, Israel remains adamant in unbelief, so God permits Rome to destroy the city in the most awful siege in ancient history, destroy the temple, and send Israel into dispersion among all the nations. That dispersion has begun to come to an end, though it hasn't ended yet for the most part. Les also notes that Israel could have repented—every Jew had to repent and be baptized—and then Christ would have ushered in the tribulation, lifted the curse, and brought in the kingdom, with Israel evangelizing the nations. But God knew Israel wouldn't do it, so He would go to the Gentiles without Israel.