The same Cicero, who affected, in his own family, to appear a devout religionist, makes no scruple, in a public court of judicature, of treating the doctrine of a future state as a ridiculous fable, to which no body could give any attention. Sallust represents Caesar as speaking the same language in the open senate.--Hume, Natural History of Religion. (12.24)
The exploits of the Athenians, as far as I can judge, were very great and glorious,something inferior to what fame has represented them. But because writers of great talent flourished there, the actions of the Athenians are celebrated over the world as the most splendid achievements. Thus, the merit of those who have acted is estimated at the highest point to which illustrious intellects could exalt it in their writings.--Sallust, Cataline Conspiracy.
The Roman historian, Sallust (especially Catilinae Coniuratio) is a regular interlocutor of Hume throughout his career; he mentions him explicitly in Treatise (here). Sallust's account of the decline of republican Rome is the only named target of his more general criticism of the Roman self-understanding (in "Of the Refinement of the Arts"):
All the LATIN classics, whom we peruse in our infancy, are full of these sentiments, and universally ascribe the ruin of their state to the arts and riches imported from the East: Insomuch that SALLUST represents a taste for painting as a vice, no less than lewdness and drinking. And so popular were these sentiments, during the later ages of the republic, that this author abounds in praises of the old rigid ROMAN virtue, though himself the most egregious instance of modern luxury and corruption; speaks contemptuously of the GRECIAN eloquence, though the most elegant writer in the world; nay, employs preposterous digressions and declamations to this purpose, though a model of taste and correctness.--Hume
Hume quickly dispatches Sallust's account: "it would be easy to prove, that these writers mistook the cause of the disorders in the ROMAN state, and ascribed to luxury and the arts, what really proceeded from an ill modelled government, and the unlimited extent of conquests." To our modern eyes, Hume's causal explanation looks more plausible than Sallust's; while the Republic was a great war-machine, its oligarchic institutions were badly designed to govern the empire it had acquired.
But it is worth noting that Hume subtly misrepresents Sallust's position. For Sallust luxury is a late stage of the decline that follows badly formed ambition, first, and then avarice: "At first, however, it was not so much avarice as ambition that disturbed men's minds--a fault which after all comes nearer to being a virtue. For distinction, preferment, and power are the desire of good and bad alike -- only, the one strives to reach his goal by honourable means, while the other, being destitute of good qualities, falls back on craft and deceit....avarice is a noxious poison." [Handford's translation slightly adapted; here's the Latin]. So, for Sallust the evils of luxury are themselves a consequence of ambition that cannot be expressed properly.
In fact, while Sallust indeed expresses himself like a moralist, his position is at bottom identical to Hume's. For he is very clear that what causes the inability of ambition to express itself to proper ends in a proper fashion is the lack of external, mortal enemies after Rome's last rival Carthage is destroyed. The very same men and institutions stop performing admirably due to changed external circumstances. Common threats in survival secure common ambitions and, we may say (according to Sallust), a spirit of unity. In fact, even in the absence of Sallust's longer work on the history of Rome (now lost), we may understand his treatment of the Jugurthine War and the Conspiracy of Cataline as a joint, extended meditation on the nature of such decline after the demise of the Carthigenian threat caused by the improper expression of ambition (of which Sulla and the vices of luxury are the degenerate consequences), culminating in civil war and implosion.
For, the rise of Marius, who is described as ambitious of power (here and here) is a consequence, on Sallust's account, of the class warfare among the nobles and the people; Marius is capable of enlisting the aspirations of the poor to his own ends. This class warfare had been a recurring theme in Roman history, but on Sallust's account, after the fall of Carthage, this class warfare came to predominate after the outbreak of peace and the lack of external fear (here). Interestingly enough, the proximate cause of the decline, for Sallust, is the disunity among Rome's ruling oligarchs who out of pride and lack of fear stop acting as a unified class. (During the rise and wars the "greatest concord" prevailed.) This opened the door to ambitious noblemen who preferred "true glory over unjust rule" (qui veram gloriam iniustae potentiae anteponerent) and in the lack of a moderate even generous response by the elite,* open-ended civil unrest.+
I am not claiming that Sallust is primarily interested in the nature of social causation in the way Hume is (although Hume hides his considerable debt to Sallust). For, it is not so much moralizing, but he nature of true glory, that is the theme of Sallust's book from the opening lines. And he clearly thinks that writing such history can also generate such glory. Perhaps, even, one of the few paths to true glory in in an age of decline.
Of course, if even Hume is not above borrowing Sallust's explanation while criticizing the man (Sallust also offers a nice example of the ills of social contagion [tanta vis morbi ac veluti tabes plerosque civium animos invaserat] that clearly also influenced Hume), then we should not be surprised that Sallust himself was not above borrowing good ideas from those he criticized. As Adam Smith is said to have remarked to his students whhile teaching rhetoric, the great speech that Sallust "put in the mouth" of Cato was itself "copied" from Demosthenes (as modern scholars agree).
+In context he is describing the Gracchi, but the culmination of the story is Marius who is also described in similar terms.
* See also the foolish, condescending comment by Metellus (in other respects an admirable general and leader).