If God knew from eternity past who would believe, does that mean we don't need to evangelize?
No. God's foreknowledge from eternity past that people would believe does not excuse believers from taking the Gospel to the ends of the earth and making everyone aware of their opportunity for salvation. The invitation remains: whosoever will may come.
Les explains that it is a false teaching to say God has already determined who will be saved and who will be lost, so that there is no use trying to evangelize because those not foreordained do not stand a chance. 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. Les connects this to Romans chapter 9, where he placed the “whosoever will” on one side and the verse about being chosen before the foundation of the world on the other, acknowledging that humanly speaking this is hard 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.