I take minimal libertarianism (ML) to entail that, for any time t, free agent S, action A, and world W, S is libertarian free at t in W with respect to A only if S can (is able to) do A at t and S can (is able to) do ~A at t. It is central to the freedom of the libertarian free agent S in W that S has available to her at t an A-world, w, and a ~A-world, w’, each of which share the same past. There are of course many complicating clauses we can add to (ML), and various resulting versions of libertarianism, but the minimal conditions in (ML) is all we will need.
Theists often find libertarianism appealing. But can theists consistently be libertarians? The standard view on gratuitous evil is in (P).
P. Necessarily, God prevents every instance of gratuitous or pointless evil.
(P) of course expresses a necessary truth (if true). (ML) also expresses a necessary truth. Both (P) and (ML) are widely accepted among theists. But if (P) is true, as it certainly seems to be, and God exists, then (ML) is false. How would a proof go?
Note first that (ML) generates a B-series of worlds. Let w be morally perfect. In a B-series, we have a world w-1 which is exactly like w except that there is one additional evil. (ML) allows that we replace a good agent in w with a bad moral agent, one who always goes wrong. There is also a w-2 which is exactly like w-1 except that we replace another good moral agent with a bad moral agent. And so on downward. The series, w > w-1 > w-2 >, . . ., > w-n, is a B-series since every world from w-1 downward includes gratuitous evil. All of the worlds have the same amount of positive value–in particular each has the same value of freedom and exercise of freedom–and the worlds lower in the series have additional evil.
Since God exists in all worlds in the series, he can replace the bad moral agent in w-1 with the good moral agent in w, and actualize a better world. He can replace the bad agent in w-2 with the good agent in w-1, and actualize a better world and so on down the series. The result is that libertarian freedom yields pervasive violations of (P). The options are to reject the idea that God must prevent gratuitous evil (P), or reject the idea that free agents are libertarian free (ML). The reasonable response is to reject (ML).