What literacy device is the author using to convey the following point of view? "In both countries it was clearer than crystal to the lords of the State preserves of loaves and fishes, that things in general were settled for ever."

The above quote is from the first chapter of A Tale of Two Cities, and it is what we call an allusion.
An allusion is a literary device which references ideas or people with political, religious, social, or literary significance. In the above, the quote is an allusion to Christ's two miracles of the loaves and fishes in the New Testament. The first miracle involved Christ feeding five thousand people by multiplying five loaves of bread and two fishes. The second miracle involved Christ multiplying seven loaves and a few fishes to feed four thousand who had gathered to hear him.
In the above quote, the French and English aristocracy were the "lords of the State preserves." They were the ones responsible for the general welfare of the people; yet, the irony is that they lived their lives oblivious to the true nature of the average citizen's struggles. While they presided over plenty, a majority of the populace in both countries lived subsistence lives. Basically, the aristocratic classes enjoyed lavish lifestyles and concluded that "things in general were settled for ever."
Additionally, many aristocrats had access to game preserves that average French and English citizens were forbidden to enter. In these preserves, the wealthy and the powerful indulged their love for game sport. The meat was not used to feed the hungry masses (hence the irony of the allusion to Christ's miracles), but to tease the palates of those who were already accustomed to good food on a daily basis. It was said that the English and French monarchy "trod with stir enough, and carried their divine rights with a high hand." In other words, the aristocrats acted like gods but failed to fulfill their responsibilities to their people.
http://dickens.stanford.edu/dickens/archive/tale/issue1_allusions.html