Let us suppose that some form of Christianity is true (or very true, or very nearly true…); in particular, let us suppose that there is such a place (or state) as heaven, where those who are good go after they die to enjoy eternal bliss. Could there be sin and/or evil in heaven? If there could, it is hard to see what could guarantee that heaven will remain heavenly. If the world is as—shall we say—imperfect as it is, with all of its miseries, cruelties, and injustices; and if this is so primarily because God has granted people libertarian free will—that is, a variety of free will that involves the ability to do otherwise than what one actually does—what reason do we have to think that heaven will be much better? Sure enough, there won’t be any really bad people there—no Hitlers, no Maos, no Stalins—but even relatively good people do bad things from time to time. So long as it is possible for saints (those in heaven) to sin, given enough time it becomes almost certain that they will do so. So if God wants to prevent the occurrence of “heavenly sins”, He will have to restrict saints’ libertarian free will. This does not imply theistic determinism: God could still allow people to choose one good act in preference to another, but He would render them incapable of choosing any bad ones. This is, as far as I know, the Catholic view (see here, section IV), and I am not sure which other Christian denominations would accept it.
Supposing it is correct, the question arises of why God doesn’t do the same for us. Why does God allow people on Earth to sin if He could prevent it? Is the answer that if He did so we would be His marionettes; that God does not want to “force us” to love Him or to do the right thing? If so, aren’t saints God’s marionettes? Is the significance of their love diminished because they cannot choose not to love God?
At this point I think a Christian who agrees with the Catholic view can make the following reply: Free will is valuable, but only in certain contexts and for certain purposes. Granted, the free will of saints is restricted, but this is not a bad thing because they did have the ability to sin during their lives. However, in choosing to accept God’s grace and do the right thing they have rejected sin. (What if you believe in “irresistible grace”? In that case you shouldn’t be concerned about libertarian free will in the first place, and you also face the problem of having to explain why a loving God wouldn’t grant His irresistible grace to everyone.) God wanted people to choose to love Him, align their will with His and reject evil freely over the course of their lives. Once they have done so, it is no longer necessary for their wills to be so free. They have proven their worth and there is no need for further testing.
One could think of it this way: Would saints want to be able to sin? If their love for God is strong, I think they wouldn’t. They might recognize that this ability is necessary for one stage of their existence, for without it they could not really choose to love God, and forced love would not really be love. Yet if they have made and confirmed that choice throughout their lives, they have no reason to want to retain this ability forever, because if they did retain it there would be a chance they could do something to damage their relationship with God, and presumably they don’t want that. In effect, saints have chosen to restrict their free will. Given that their lives involved accepting God’s grace, doing good and choosing to love God, why wouldn’t God honor their choice? Even if they don’t consciously make that choice, we may suppose that God will restrict their free will just in case He sees that that is what they would choose if they explicitly considered the matter.
Thus the reason why people have unrestricted libertarian free will on Earth is that God wants our love of Him to be genuine, and thus for us to be free to accept or reject Him—for a time. Such, at any rate, runs one possible Christian reply. I leave it now to my readers to see if they think it succeeds.