My position on the role of what motivation we might have to evangelize if Calvinism is true has, admittedly, changed a bit since I first blogged about it.
My position is this:
1) Calvinists certainly have the motivation to evangelize based on the Great Commission.
2) Calvinists have the motivation to evangelize based on a desire they might have to play an instrumental role in someone's coming to receive saving faith. (This is what I had left out.)
3) It is not clear that Calvinists have the motivation to evanglize based on the truth of certain subjunctive-conditional claims concerning what the oucome will be if they do or do not evanglize.
The reason is that, given what God has predestined, the future is closed. Suppose Smith witnesses to Jones and Jones is saved. Smith, however, has struggled with getting up the courage to witness to Jones. He wonders if it will make a difference as to Jones' salvation whether he preaches or not. He knows that his preaching will not cause Jones to become one of the elect, since the elect were chosen unconditionally before the foundation of the world. Can the statement "If I don't witness to Jones, Jones won't be saved" be true if in fact Jones has either been unconditionally elected or unconditionally reprobated. What would make such a statement true or false? Any world in which Jones doesn't witness is a world which God did not predestine. Asking the counterfactual question is assume that there are other possible worlds, but there are no other possible worlds. Ultimately what you are asking if you are asking the counterfactual question of "What would have happened if I had not witnessed" is to ask "What would God have predestined to have happen to Jones if God hadn't predestined that I should witness to him?" I can't see how to make sense of the statement "In the nearest possible world in which I don't witness to Jones, Jones is reprobated." Is there even such a possible world?
It seems an Arminian can say that by sharing the Gospel with Jones, he makes it more likely that the actual world is a world in which Jones is saved. If Calvinism is true, I don't see how you can say that.
Should Calvinists care? I said earlier that Calvinists need not see this as a problem for their position. When I am making an argument against Calvinism, I will tell you.