Question:  I’m struggling with 1 John 2:2 (He is the propitiation for our sins, and not for ours only but also for the sins of the whole world).  If Jesus was the propitiation for the sins of the whole world, why then is the cross not effective for the whole world?

Just because something is offered doesn’t mean it will be enjoyed.  For example, my wife may cook a wonderful meal with five different recipes that I think are all good.  However, my daughter may choose to eat only three of the five recipes.  Did Wendy prepare enough for our whole family?  Yes.  Did every member of our family eat all five dishes?  No.  Does this make Wendy’s cooking ineffective (I’d better be careful here!).  No!

Effectiveness is not determined, in salvation, by how many receive it.  Jesus death on the cross was sufficient for all, but only efficient for those who respond in faith.