Perhaps the most important of these is his sharp criticism of Hume's distinction between superstition and enthusiasm, and of Hume's fairly extensive use of these concepts to explain historical events. Hume tends to treat superstition and enthusiasm as diametrically opposed: superstition likes rigid law, enthusiasm likes anarchic liberty; superstition involves excessive self-doubt and heteronomy, enthusiasm involves excessive self-confidence and autonomy; superstition is more society-friendly in the short run but leads to more violent excesses in the long run, while it is the reverse with enthusiasm; and so forth. But as MacQueen notes, it isn't clear why they are treated as mutually exclusive:
These two species of religion (to use his style) ar eevidently distinct the one from the other : but they do not appear to me to be "diametrically opposite." I can perceive no absurdity in supposing, that one may embrace the tenets, and practise the rites of superstition, who notwithstanding may be possessed of no inconsiderable portion of the fanatical spirit. Nay, I can easily imagine a plan of religion, which, in some of its doctrines and institutions, may be extremely favourable to superstition; in others again, to fanaticism. (p. 21)
Not only can he imagine it, he's certain that it actually exists, in the Catholic Church; and much of MacQueen's argument is that Hume is wrong to identify Protestantism with enthusiasm and Catholicism with superstition. On the contrary, Protestantism is the golden mean between the two extremes of Catholicism and Catholicism!
But MacQueen goes on to argue that Hume's characterization of enthusiasm is ambiguous in a number of its features. For instance:
Intrepidity of spirit is the concluding article of proof in the above paragraph; concerning which every one knows, that as it is grounded in the natural constitution, so it may be strengthened in different persons, by various principles and motives that are suited to affect them. A sense of duty, a sense of hnour, a sense of religion, each of these will, in its turn, confirm it. Where the cause is good, intrepidity of spirit in its defence is an honourable distinction. But boldness and intrepidity may be likewise exerted in a quite opposite cause, andi n prosecution of the worst purposes. In the concerns of religion it may be exerted by the inflamed bigot, and the wild enthusiast, as well as by the wise and worthy friends of truth. here then is another supposed mark of enthusiasm, which is in itself wholly ambiguous,a nd from which therefore no conclusion can be drawn. (67-68)
But, you will say, was there nothing intemperate in their zeal, nothing irregular in the manner of their prosecuting the great ends they had in view? Who ever affirmed that there was not? They were men; consequently neither infallible nor impeccable. But then, my friend, intemperate zeal is not enthusiasm. They are no less distinct than effect and cause. Neither does the effect here follow necessarily from the cause; for we have many a time heard of harmless enthusiasts: nor is it an effect of this cause alone, as it may be at least equally derived from credulous and blind bigotry. Nay, I will venture to affirm that furious and desolating zeal hath been found in alliance with the superstitious principle more frequently than with the enthusiastic one; and that its ravages, when thus allied, have been more deplorable and dreadful. (72)
In this passage I think MacQueen comes very close to seeing something I've noted before, namely, that Hume seems to be trying to make these concepts fill two roles, one psychological and one sociological, and it is difficult to see how they can do both, and this leads to occasional apparent contradictions in which enthusiasm in its psychological role (excessive confidence in one's link with God) makes alliance with superstition in its sociological role (priestcraft and rites and establishment religion), or superstition in its psychological role (excessive reliance on rigid principles) jumps together with enthusiasm in its sociological role (no authority); or when he apparently attributes the melancholy of superstition to enthusiasm; or some such. It isn't clear that these are actually self-contradictions; but they do seem to be signs that Hume isn't adequately clear in his use of these concepts to build historical explanations.