Why was God going to do something totally different after Israel rejected the message?
Because Israel rejected God's offer in unbelief again, just as at Kadesh and many other times. When they refused His message, God said He would do something different, which is why He raised this apostle and brought in salvation by grace through faith.
Les teaches that seven or eight years after Pentecost, the scattered Jews had preached the Word to none but Jews only. They had gone as far as Antioch in Syria and other places, but still only to Jews. Then God was going to do something totally different because Israel was rejecting the message in unbelief again, just like at Kadesh and many other times. Since they rejected God's offer, God said, in effect, that He would do something different. This is why He raised this apostle. Les then connects this to Ephesians 3 and Ephesians 2:8-9, where salvation is by grace through faith, not by works.
Now they which were scattered abroad upon the persecution that arose about Stephen travelled as far as Phenice, and Cyprus, and Antioch, preaching the word to none but unto the Jews only.