The following is a quick sketch of an argument against purgatory based on other things I have argued. To accept purgatory implies, at a minimum, that after death there is some place not identical with hell and not identical with heaven where one will continue to morally develop. If one becomes morally perfect, then one leaves purgatory and enters heaven.
An argument for purgatory runs as follows: necessarily, one’s will must have an independent (i.e. non-determining) causal role in making oneself as a moral agent a morally good agent. But by the time most of us die, we are not perfectly morally good. In order to be in heaven, one must be perfectly morally good. So there must be a place in which one must become morally perfectly good before one can enter heaven. And we call that place purgatory.
The problem with this argument is the first premise. The argument against purgatory runs as follows: It is possible that God creates humans good without their wills playing any role in making them good. It is also possible that God regenerates fallen humans so that they are morally better than they were at a prior time without prior approval (or non-rejection) of their wills. Therefore, it is possible that God makes humans morally perfect without the independent (i.e. non-determining) causal role of their wills. Hence purgatory is unnecessary.
The argument for the claim that it is possible that God creates humans good without their wills playing a role is presented in the problem of just creation. The argument for the claim that it is possible that God regenerates fallen humans so that they are morally better than they were without prior approval is presented in the problem of harmonization. Both problems are presented in my paper, Anselm on Freedom and Grace. The inference to making humans’ wills morally perfect after (or at) death is by analogy of the cases: being made a creation for the first time, being made a “new creation”, and then being a glorified creature.