Did God immediately start working through the Gentiles after Israel rejected Christ?

No. After Israel rejected and crucified Him, God did not immediately turn to the Gentiles. Scripture shows He continued dealing with the nation of Israel in the early chapters of Acts, until the Body of Christ, the Church, was introduced through the revelations given to the Apostle Paul.

Les teaches that even though everything was consummated for the salvation of the whole human race, God did not immediately go through Gentiles after Israel rejected and crucified Him. Instead, He continued to deal with the Jew in the early chapters of Acts. This continued until the Body of Christ, the Church, was introduced. Les points out that the term “Body of Christ” is Pauline terminology and is not seen until the revelations given to the Apostle Paul, such as at his conversion on the road to Damascus and his encounter with Ananias.

Did God immediately turn to the Gentiles in early Acts, or did He continue dealing with Israel?

In the early chapters of Acts, God did not immediately turn to the Gentiles. He continued dealing with the nation of Israel, even after the crucifixion and the completion of salvation for all people. Only later, through the revelations given to the Apostle Paul, is the Body of Christ, the Church, introduced.