The opening two paragraphs describe the condition in England and France in 1775, the year the novel begins, establishing this as a historical novel (it was published in 1859). Dickens points out that the condition he describes is very much like the "present period," or his own times, too, universalizing...
Therefore, using a series of antitheses or oppositions, Dickens describes what times are like universally, but also, more particularly, what they were like in Britain and France in 1775, fourteen years before the French Revolution. Times are the "best" and, simultaneously, the "worst." They are filled with wisdom, belief, light, and hope but also with foolishness, incredulity (disbelief), darkness, and despair, all intermingled. Using anaphora, or repetition at the beginning of lines, Dickens establishes a sense of litany, giving his opening the timeless quality of religious authority.
From the start, Dickens is setting up the oppositions that will define this novel about the French Revolution. While it might seem, too, that Dickens is setting up an opposition between London and Paris, he is at pains to show the similarities between both cities, each filled with corruption, darkness, and despair. The opening anticipates such themes as the antithesis between the hopes and realities of the French Revolution, as well as the way life can simultaneously look very differently depending on where you stand on the social, economic, and political ladder. From this opening, we can expect the novel to be a broad commentary on society and social ills.