Using tendentious argumentation and sometimes untenable, rejected (cf. “Forged!”), sometimes sensationalist, theories (although he’s an order of magnitude better than that other popular Bible “scholar”, Elaine Pagels). He’s decent at introducing the field, but does it in a distinctively critical - in the sense of “contra”, not “critical thinking” - way, and is very uneven, not introducing a basis and building on it as one would be taught in a class, but introducing only those parts viewed most sensationally by the general public (like “Misquoting Jesus”, “Forged”, “Lost Christianities”, “Lost Scriptures”), and he loves to redefine words and concepts that are already defined in the academic community - i.e. calling gnosticism and other heresies “lost Christianities”, when it’s been called gnosticism for 1800 years, with other appellations, such as Valentinian, Sethian, etc. for specific branches, which he doesn’t do well with. He’s fast and loose and very muddled: not as bad as the Jesus Seminar or Elaine Pagels (both less scholarly and more sensationalist), but nowhere near a heavy-hitter.
He may be one of the most popular “popular science” Bible scholars alive, but he is in no way in the Big Leagues of RE Brown, Bruce Metzger, William Farmer, FF Bruce, or any number of other scholarly researchers. As I mentioned above, Elaine Pagels is probably the most popular “popular science” Bible scholar alive, or close - if she’s not, Bart Ehrman is - but she’s not even a scholar - more a yellow journalist than an academician. More people have probably heard of Geza Vermes (translated Dead Sea Scrolls with controversial commentary) and Marvin Meyer, or whoever the guy was who translated the most popular edition of the Nag Hammadi library (the one with “child of humanity” instead of “son of man”) than have heard of Bruce Metzger or Raymond Brown. More people have heard of and read Dan Brown, who presents himself (whether he actually did, or whether it was all part of the novel/publicity stunt, I don’t know: I think the latter, but many people thought the former, and bought in to his tripe) as the scholar par excellence, single-handedly breaking down aeons of dogma, than all of the above combined. Michael Baigent (Holy Blood, Holy Grail, the inspiration for Dan Brown) is probably as well-known and widely read as Bart Ehrman or Elaine Pagels, and, like Pagels but to an even greater degree, he is an unabashed conspiracy theorist - at least Pagels cloaks her tendentious sophistry in the guise of Christianity-cum-gender-studies.
Popularity doesn’t mean correctness, or even proficiency. One only need look at politics to see that.