If God knew in eternity past who would believe, why do we still need to evangelize?
Because God's foreknowledge of who will believe does not remove the command to carry the Gospel to the ends of the earth. Evangelism makes everyone aware of their opportunity for salvation, and Scripture holds together divine election with the invitation that whosoever will may come.
Les explains that some people twist God's foreknowledge into a fatalistic false teaching, claiming God has already determined who will be saved and lost, so there is no use trying to evangelize. He says that is not what it means. God knew from eternity past that we would be believers, but that does not excuse us from taking the Gospel to the ends of the earth and making everyone aware of their opportunity for salvation. He connects this to Romans 9, where the "whosoever will may come" stands alongside being chosen before the foundation of the world, and he acknowledges this is humanly difficult to understand.
According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
More Questions
Is it true that God already decided who is saved and who is lost, so there's no point in evangelism?
It is a false teaching that God has already determined who will be saved and who will be lost so that there is nothing you can do about it and no use evangelizing; that is not what being chosen before the foundation of the world means.