Like a number of Shaw's plays, he begins this one with an introduction on the topic of the play, which seems to be about the nature of good and evil and how in reality it is quite difficult to distinguish between what is good and what it evil, especially in the political sphere. The play is set during the American Revolution and is about how 'the Devil's Disciple', Richard Dudgeon, a self-avowed devil worshipper, takes the role of the village vicar in order to save the other's life.
Shaw opens his introduction with the statement that while the gates of heaven may be accessible from the mouth of hell, so the mouth of hell is accessible from the gates of heaven. The meaning of that, initially (that the gates of heaven are accessible from the mouth of hell) is that despite one being what some consider to be 'damned' nobody is truly damned while they still have breath in their body. What that may be true (it is, as far as I am concerned) Shaw also points out that (as far as he is concerned) salvation is never really assured. Okay, I feel that the Bible does contradict that in a way, but from what I understand what he means is that there are a group of people who are so confident of their salvation that they may not realise that their actions on Earth are actually having the opposite effect on them (and Jesus pointed to the Jewish ruling class of his day as example of this – though once again he also implied that their fate had not yet been set in stone).
Another interesting thing that Shaw brings out (other than the stupidity of writing an introduction to his play telling people what his play is about because painters don't do the same thing with their paintings) is that how interpretations of a play may change over time (which is while he suggests that the introduction is silly because while he may have one interpretation of the play, others will not necessarily have the same opinion). What Shaw understands is that thoughts perceptions, and culture changes, and as such what may be popular in Shakespeare's time, and what may be accepted in his time, may not be the same as it is in Shaw's time, as is the case in our time. Thoughts and ideas change, as does the idea of what is good and what is evil. There was a time when homosexuality was a crime punishable by gaol time, however that is not the case now (and has not been been for over thirty-five years). On the other hand, there was a time when smoking cigarettes was perfectly acceptable, whereas now, if you smoke in the wrong place you can be thrown out, or even prosecuted.
Shaw also raises the point of the nebulous nature of good and evil by setting the play during the American Revolution (or whatever name you may give it, though I suspect using the word rebellion is probably not the best term since a rebellion suggests that the armed uprising failed, while the term revolution suggests that it succeeded, so as much as some people may want to, pardon the cliché, call 'a spade a spade', the truth is that the American revolt against Lord North and King George succeeded, meaning that it was in fact a revolution as opposed to a rebellion – though let us not go down the road of defining freedom fighters, I might get myself in trouble). The reason I raise this is not just because it involved Anglo-saxons fighting Anglo-saxons (there are plenty of other wars where that occurred) but because both sides had their own reason for fighting the war, and both sides saw their respective goals as being 'good'. The British saw it as defending the integrity of the empire, as well as putting down a rebellion against the rightful rulers (once again something that is open to definition) while the 'Americans' were fighting to protect their right to self-determination, and against what they considered to be an unfair taxation system. Interestingly enough, if California today were to revolt against the Union (as was the case during the American Civil War) both sides would more likely than not use similar statements with regards to the rightness of their position (though because the South supported slavery, people don't like to suggest that they were right, but in reality, the reason that the South rebelled against the North during the Civil War had more to due with undue influence of the federal government over the states). The same can be said of the current unrest being experienced in the Ukraine, where the Eastern portion does not want closer ties with Europe, but prefers closer ties with Russia, where as the Western majority consider themselves to be European.
So, in the end, what we see here is that we cannot necessarily label somebody as good or as bad based simply upon our own thoughts and opinions. Everybody considers Dudgeon to be bad, the 'Devil's Disciple' as many people, including himself, put it, yet he goes out and takes the place of the town vicar, acting in a very Christlike manner, which is something not to be expected by a 'Devil's Disciple'. Rather, one would expect that he behave in a selfish manner, selling out his friend's and family for his own life, yet this was something that he does not do. As such, the notion of good and evil, or war and peace, is turned upside down.
By the way, a film version is available on Youtube.