From Richard Popkin's obituary in the UK Guardian:
The History Of Scepticism revolutionised the received picture of both the history of philosophy and the history of science, by demonstrating the influence, in the century before Descartes, of ancient Greek sceptical arguments about the impossibility of knowing God and the world.From Popkin Non-Scepticus by Brian Copenhaver, which in turn is from The Legacies of Richard Popkin, edited by Jeremy D. Popkin:
In making his case for this central contribution to the development of modern science and philosophy, Popkin gave attention to the intellectual context of the time, especially the role of religious disputes in the take-up of philosophical scepticism deriving from the discipline's Greek founder, Pyhrro. Instead of treating the history of science and philosophy as a series of breakthroughs by canonical figures, Popkin sought to view the thought of the past from within its own framework.
His history brought him international recognition and was translated into four languages. He expanded his thesis in later editions of the book (most recently in 2003), and in The High Road To Pyrrhonism (1989), which took the story through to David Hume. His interest in the contribution of non-philosophical strands (especially religion) to the history of philosophy led to pioneering studies of the interaction of Jewish and Christian philosophy and theology, and of topics such as kabbalism and millenarianism.
Part of the force that drove Popkin's monumental achievement was human and social - his dazzling gift for talking to people and convening them for hundreds of projects, conferences and publications of great and enduring influence. But what he achieved intellectually is deeper and wider than that. His range was enormous, of course, centering on French, Dutch, English and Jewish thinkers of the seventeenth and eighteenth centuries but reaching forward to twentieth century politics in this country and back to fifteenth century religion and philosophy - to the Italian Renaissance. In fact, his influence on the historiography of Renaissance philosophy was profound.From A Rebours: Richard Popkin's Contributions to Intellectual History by Allison P. Coudert, also from the same volume: The Legacies of Richard Popkin:
Popkins most important book - one of a multitude - is his History of Scepticism.... The first edition, Popkin writes, "was submitted to two major academic presses ... [but] turned ... down on the grounds that it was not sufficiently philosophical"; it appeared nevertheless in 1960, followed by a second edition in 1979 and a third in 2003. Evidently it was sufficiently readable.
Popkin's very compelling story, according to the title of the first edition, goes From Erasmus to Descartes; but the second edition goes From Erasmus to Spinoza; and the third From Savonarola to Bayle. Once the apocalyptic Florentine friar appeared in Popkin's title, the Italian Renaissance of scepticism had finally made the headlines of Anglophone historiography. That alone was newsworthy, given the previous record of oblivion, both for Renaissance philosophy and for scepticism.
If you still need help forgetting the Renaissance, read almost any history of philosophy written in English before Schmitt's Cambridge History became influential. One such work, first published in 1914, was still in print when Popkin was teaching at Iowa and Schmitt was studying at Columbia; this was A History of Philosophy by Frak Thilly, a Kantian who taught at Berkeley. In a book of 677 pages Thilly's Renaissance rates fewer than two dozen, including one whole paragraph on scepticism - mainly on Montaigne.
Otherwise, scepticism was chiefly a Greek affair for Thilly, and thus stuck in antiquity. In the modern period, Berkeley gets just one paragraph to refute it. Pierre Bayle gets twice as many to expose inconsistencies in religion and work his "potent influence on Hume." but Thilly's Hume is the Third Person of the British Empiricist Trinity and thus immaculate against such stains. Hume has his doubts about cause and effect, of course, and about knowledge of the external world and other such items, but we are not told that these worries are "sceptical." the word enters Thilly's main account of Hume only in an affecting digest of the Dialogues Concerning Natural Religion, where "in spite of these skeptical reflections, Hume declares that it hardly seems possible that anyone of good understanding should reject the idea of God .... How seriously these remarks are to be taken .... the reader is left to decide for himself."
That was where scepticism stood in the awareness of Anglophone philosophy when Popkin awoke it from its dogmatic slumber.... [pp. 5-7]
I met Dick at the Clark Library in 1984, where I worked up the courage over several days to give him offprints of two articles. At that point I had no reason to think that Dick would be any different from most accomplished academics, someshat loathe to take handouts from unknown scholars, especially ones working on esoteric subjects like Kabbalah and witchcraft. But, as I quickly learned, Dick's skeptical and inquisitive bent inclined him to question prevailing wisdom and Whiggish interpretations when it came to understanding the past. Like two other great scholars of the twentieth century, Gershom Scholem and Frances Yates, he saw himself as something of an archeologist, digging deep in what he has described as the "marvelous and varied intellectual world or swamp which lies beneath our present thinking" to ferret out little known figures, whether they be neglected persons from the past or unrecognized scholars of the present. It was at the margins, in the writings of ignored and neglected figures, that Dick found ideas now seen to be central to our understanding of the transition from the early modern to the modern world....[P.S. I'm still working on an overview of the trial of Socrates, but it might be a day or two before it's ready.]
In thinking how to define Popkin's contributions to scholarship I was struck by the family resemblance that exists between his work and that of two other distinguished scholars I referred to earlier, Francis Yates and Gershom Scholem. In significant ways all there were "heretics" inasmuch as they went against the grain of accepted scholarship by emphasizing the centrality of what other scholars had marginalized, denigrated, or ignored. As I mentioned, Dick was fascinated by what he referred to as the "swamp which lies beneath our present thinking." Scholem had a similar penchant for delving into uncharted regions. He was convinced that one had to excavate traditional history to get to the truth hidden below the surface, and he discovered sources of this hidden truth well beyond the borders of orthodoxy....
Yates shared this same conviction that true history was subterranean. Like Popkin and Scholem, she saw herself as an archeologist, whose excavations among the ruins of the past revealed the truth that lay beneath what she described as "superficial history." ... Yates pursued the theme of "lost" history throughout all her work....
When one thinks about the factors that drove Popkin as well as Scholem and Yates to direct their historical investigations to areas beyond the borders of orthodoxy, biography becomes important. In his two autobiographical essays Popkin describes himself as by nature "rebellious." He rebelled against his parents' dogmatic liberalism, anti-religion, and communist world view. This rebelliousness continued at Columbia, where he rejected John Dewey's intrumentalism and Frederick Woodbridge's naturalism. It wasn't until he discovered Sextus Empiricus that things began to fall into place. As he says in a passage that makes both Francis Bacon and Karl Marx jump to mind:
In my own case, I guess that I feel perpetually an outsider and an outcast ready to smash intellectual idols at any time. An intellectual anarchist might describe this view, who feels the common human bond would be revealed if our intellectual chains were broken and our deceptive glasses removed. Theories would be seen as myths with no supra-human dimension. Only the supra-human experience found in religious experience and aesthetic experience transcends this. But any interpretation puts one back in Plato's cave looking at shadows and illusions.[pp. 15-20]
The Varieties of Sceptical Thought:
 Sceptical Scepticism
 Socrates on Death
 Socrates' Advice to the Jury
 Richard Popkin
 Overview of the Trial of Socrates
 "Socrates of all mortals is the wisest"