Why did God raise up Paul as a new apostle when Israel rejected the message?
God raised up Paul as a new apostle because Israel rejected His offer in unbelief again, just as they did at Kadesh and many other times. Since they refused the message, God said He would do something different, namely salvation by grace through faith (Ephesians 2:8-9).
Les explains that seven or eight years after Pentecost, the scattered Jewish believers were still preaching the Word to Jews only. Israel then rejected God's offer in unbelief again, just as at Kadesh and many other times. Therefore God said He would do something different, and that is why He raised up this new apostle. Les ties the "for this cause" in Ephesians 3:1 primarily back to Ephesians 2:8-9: salvation by grace through faith, a message no one had heard before.