According to Leibniz, we live in the best possible world. Imagine some possible world w1 in which the cumulative best state of affairs is instantiated. This means that w1 is the best possible world, regardless of whether w1 is the actual world or not. Now consider the following argument:
1. The best possible world w1 is actually a possible world. (Premise)
2. Necessarily, w1 is possible if and only if a maximally great being exists in w1. (Premise)
3. Hence, a maximally great being possibly exists. (From 1 and 2)
4. Therefore, a maximally great being exists. (From 3 and S5)
(2) seems obviously and intuitively true to me, but I'm interested in how a proponent of the argument might argue for its veridicality. If the world would be a better place to live in if there were a God (a maximally great being), then the best possible world would have to have a God, would it not?