If God already knew who would believe, does that excuse us from evangelizing?
No. God knew from eternity past who would be believers, but that does not excuse anyone from taking the Gospel to the ends of the earth and making everyone aware of their opportunity for salvation. Foreknowledge is not the false teaching that God has already determined who is saved and lost with nothing anyone can do.
Les explains that Ephesians 1.3 says we were chosen in Him before anything was ever created. That truth is mind-boggling and throws a curve at many people, and some take it into a false teaching that God has already determined who will be saved and who will be lost, so there is nothing anyone can do and no use trying to evangelize. But Les says that is not what it means. God knew from eternity past that we would be believers, yet that does not excuse us from taking the Gospel to the ends of the earth and making everyone aware of their opportunity for salvation. In Romans 9, Les taught the tension between “whosoever will may come” and being chosen before the foundation of the world. Humanly speaking, it is hard to understand, so we should not simply nod as though it is fully explained.