Often it takes an illustration to help non-Calvinists break out of their categorical and cultural assumptions about the precious truth of predestination. The following illustration has floated around in Calvinist circles for many years, and has been very effective:
You have 100 criminals on death row. They are all equally guilty and deserving of death. Every single one of them hates the governor of the state. So much so, that all of them conspired together and successfully killed his only son. One of these death row criminals is you. The governor has the freedom and right to pardon and give clemency to any of them. It could be one person, ten, all of them, or just none of them. If he chose to pardon none of them, would he be perfectly just to do so? Yes. And he is not obligated to choose between a dilemma of bestowing mercy either on all 100 of them, or none of them—he could choose any number in between, if he wills. He can do whatever he wishes because of his right as governor. But let us say he chooses to have mercy on 10 of the 100 justly deserved death row criminals. The ten are just as guilty and deserving of death as the other 90. And one of those ten to be graciously pardoned is you. You are free! You are pardoned! You have been granted mercy! Now as you are stepping out of that prison into freedom, are you going to look back and point your finger at the governor and utter, “How dare you pardon me, and not everyone else.” You would be an ungrateful halfwit.