In a comment, a concerned reader named Esther O'Reilly asked me how dumb I think first century Jews were. I think this is a legitimate question that deserves an answer.
A little background: The question was a response to my statement that I don't think it's surprising at all that early Christians and the Jewish communities depicted in the Bible would define "faith" as "trust (in God)," because they were still superstitious enough to think that deities and magical powers abounded, lacking better explanations for things. (I know, we're still that superstitious, but it's pretty much beyond debate that we're far less superstitious now, even in those regards. Many of the ancient superstitions that served us before we had science have been replaced by scientific explanations.)
Getting our bearings
So, the answer is that I don't think first century Jews were dumb, at least no dumber than people now, and I don't think people now are all that dumb either. Particularly, I don't think intelligence has anything to do with the matter. It's all about tools.
We'd do well to remember throughout this conversation, though, that until only quite recently, relatively speaking, letting leeches suck the blood out of someone was considered a standard element of medical care for a wide variety of ailments. This was based upon superstitions about how the body works--humours--that persisted for at least 2500 years, largely ending maybe a century ago, give or take a few decades. Except for a few legitimate applications in which their anti-coagulating properties were used to help with some surgeries, mostly in the 1980s, medical science revealed the superstitious nature of bloodletting and led to the end of the medical application of leeches. The point here, of course, is that we were largely clueless about how the body works, had a superstition, applied it in consequential cases, and persisted in it until quite recently when science revealed it to be worthless at best and probably a generally bad idea. People are superstitious, and without tools like science to do better by understanding the world, superstitions proliferate and perpetuate.
Looking around at today, well after science has come to a state of relative maturity and dispelled a huge proportion of antiquated, superstitious thought (it's now considered profoundly backward and ridiculous to seek leeching as a form of medical care by nearly everyone, for example), we still see a lot of superstitious behavior and belief. Grown adults are afraid of black cats for no good reason, the origins of this being in ancient Christian beliefs about witches and the Devil. All manners of hocus-pocus "alternative medicine" and trendy dietary guidelines are attended to with what we could definitely call religious zeal. People still wear, and refuse to wash, lucky underwear because sportsball teams are believed to do better for it. Oh, and yes, we're still bizarrely, almost inexplicably, religious. And this is all after science has done its work to cut away a huge swath of our superstitiousness, perhaps decimating it from pre-scientific times or more.
Frankly, people in the first century were, as examples of the animal species Homo sapiens, largely indistinguishable from people today. On average, they were just as smart, measured by intelligence, as people are now, and that intelligence was trained on the challenges of the day and trained by the tools they had at their disposal. One tool we have that they didn't is science, and that's very significant. Science is the first truly effective tool that we've developed for cutting to the heart of the matter of how nature, ourselves included, works. In the process of the explosion of scientific thought, as it progressed, save the dominant religions of the day, gods were utterly ruined by the findings of science. They weren't needed to explain lightning, weather, flooding, crop yields, infant survival, or almost anything else. Demons fell away too. They were superfluous to understand disease, geologic catastrophes, bad weather, mental illness, and almost everything else.
First century Jews didn't have this tool at their disposal. Guess-and-check, which is a very weak and informal way to think quasi-scientifically, was their best tool, and the problem with it is that it's immensely susceptible to biases, particularly confirmation bias. This is where superstitions come from (and, incidentally, hypotheses consistent with evolution can easily attempt to explain why it would be advantageous for a sentient animal to become superstitious--this being the key flaw in Alvin Plantinga's famous Evolutionary Argument Against Naturalism: superstitions prove that we're very, very good at getting wrong but useful answers to how the world works).
Lacking science and possessing a plethora of mythological and superstitious accounts for the workings of nature, first century Jews weren't stupid, they were misinformed. They possessed models of the working of the world that would lead them to believe certain kinds of stories readily and to make attributions to gods, demons, fates, djinns, and all manner of other imaginary causes for the phenomena they experienced on a day-to-day basis, and even more for rare but significant events (like the ascension of a king or emperor to a throne, a decisive victory in a battle, or the arrival of a figure or movement who upsets the prevailing social order).
Another important fact about Jews in the first century, and critically in the century before that, is that they were living in a sort of political oppression under Roman occupation. This is exactly the kind of extraordinary outside pressure that increases belief in desperate hypotheses. It's like when someone gets cancer and suddenly starts believing that acai berries have magical powers to cure diseases--the stress of the situation and desperation of the circumstances leads people to cling to potential sources of hope. The Jewish people traditionally were ones who lived in a brutal sociopolitical region filled with turmoil, occupations, genocides, and the like, and so their superstitions contained lots of desperate ideas about a savior-king, a Messiah, who would deliver them from their plight--a recurring theme throughout the Jewish portion of the Bible.
So we had people who were superstitious enough to make magical attributions for phenomena as a matter of course, lacking the requisite tool (science) to do better with a history of a certain kind of desperate hope who found themselves under somewhat serious political oppression. Those people were first century Jews, and they were ripe to believe certain kinds of things. The (fortuitous? maybe not) arrival of an unexpected social movement led by a political dissident, one who was charismatic enough to gather a band of followers through his public lectures, was exactly the kind of event that a whole lot of superstitious baggage ("prophesy") could get unloaded upon, particularly in the dissonance-reducing phase after the incongruously inglorious fall of their charismatic leader.
Superstitious people who believed versions of this particular story started writing it down a few decades later (after it became clear that Jesus' assertion that he was coming back to start a new Kingdom wasn't going to happen), and as time progressed (including both historical events and the usual development of legends), the stories got more superstitious. The Gospel of John is so much more superstitious than that of Mark that it isn't even taken in the same family as the other three and is often regarded, even by many biblical scholars (but not the most conservative ones), to be a Christian manifesto more than an account of real events. It's often forgotten that the Gospel writers were writing to specific audiences to convince them to believe a certain way, but that's really how that was.
In short, intelligence being completely irrelevant, many Jews in the first century were ripe to believe a story about a figure like the one now recognized as Christ, and some did, but that's not the whole story.
First-Century Romans (and others)
It wasn't just Jews in the first century but also the Romans that surrounded them (and Greeks, among others) that are relevant here. It's very important to realize a great many of the facets of the story of Jesus were ideas that were not considered outlandish in the ancient world: stories of virgin births of important figures, particularly from maidens impregnated by deities, for instance, were rife. No part of the Jesus story would have seemed particularly outlandish, so far as stories about important figures went, to a great deal of the audience at the time. It's critical to realize this, and it's important to understand that some, but not all, is all it takes to get a movement rolling.
This would explain the Gentile conversion, particularly under the tutelage of the zealous ideologue Paul--Saul of Tarsus--who sought to convert people to his hyperreligious position "by any means." That his hyperreligiosity, visions, etc., could not be adequately explained at the time, as they could probably be now (e.g. perhaps by epilepsy in the temporal lobe), his attribution of his conversion and manner to a Spirit wouldn't just have been in line with contemporary thinking, it would have been powerfully persuasive. (One might note that it still is with a large segment of people today when we really should know better, Paul's audiences not having nearly so much of that luxury.)
In the ancient world, superstitions were in competition. The Bible is a veritable tome of competing superstitions, and like surely happened through the story of Elijah in 1 Kings 18, he with the story of the better magic got converts. At first the Romans and Greeks, among others, had no real use for the Jewish or Christian story, save on the small scale (the growing cult of Christianity that was tolerated for a time and then persecuted). Paul could only convert small groups, and by some of his angry Epistles, he clearly had trouble with the affair. As it turns out, even ancient superstitious people weren't immediately gullible to any story that comes along, and they were certainly not stupid.
This story cannot be completed without talking about Constantine, because he made Christianity. Again, the populace at large is relevant.
Romans were still superstitious, but the Roman Empire was having problems (largely due to political corruption due, I would venture). The Roman gods, the predominant Roman set of superstitions of the day, were starting to look like pretty bad explanations for things, as gods always do once things start going badly. (Incidentally, gods tend to look like worse explanations for things, often, when things go really well, so long as you have a tool like science to cut away the need for many superstitions.)
So now you have a people, the Roman people, whose superstitiousness was still as high as ever, maybe higher given the decline of their situation, whose superstitions were faltering. You also had a large and significant Christian movement that had been inadvertently strengthened by the attempt to persecute it out of existence. This was creating a major problem for Constantine, who solved it with what might be one of the more deft political maneuvers in all of history. Certainly, that's not indicative of a stupid person.
For those unaware, Constantine came up with a way to make Christianity the state religion while effectively making it seem to the pagans that he hadn't really changed anything except which pagan construct deserved most of their attention. As their operating emperor, which is to say earthly incarnation of deity, changing the focus of the pagan beliefs wouldn't have been overwhelming, perhaps even matter-of-course, and Constantine knew it. Meanwhile the Christians benefited hugely, not just socially but also politically as the bishops immediately gained considerable political power that even outlived the Holy Roman Empire and became the basis for the future realm referred to as Christendom, one of the most potent political forces the Western world has ever been under the control of. It's worth noting that had Constantine made his decision differently, we might be having this conversation about Mithra instead.
Once Christianity became politically installed, there's almost nothing regarding intelligence that has anything to do with belief in the ridiculous Christian precepts. They were the fabric of the worldview pressed into every mind under its thumb, and shaking off an entire worldview is really, really hard.
So, Not Dumb
Hopefully, what I've done here is make a plausible case using a rough historical outline of circumstances for why I think it wouldn't have been ridiculous for ancient people to believe in things like God. If one believes in God with the whole fiber of his being, even if God is not there, it isn't patently ridiculous to believe that one can trust God. All that said, the first century Jews, first century Near Eastern non-Jews, and up through the fourth (and nineteenth and present-day) people didn't believe these ridiculous things because they were dumb, they believed them because they were superstitious and, later, raised to believe them. The difference now is that we have a tool, science, and a body of knowledge procured through it that makes it look quaint, antique, superstitious, and silly to buy into that stuff now--though billions still do.
So, no, I don't think Jews in the first century were dumb, but to answer the question I was asked--how dumb do I think they are?--I have to reply, "roughly the same as us."
Science, a reliable method for picking apart what's going on and how it works, has changed the game completely. We don't blood-let anymore. The educated scoff at astrology. Nobody is an alchemist anymore except a crank here or there. But lots and lots of people believe that if you believe in the right kind of magic, you're automatically a good person and get to live forever in a paradise. What's going on?