I, along with most scholars of the NT I am familiar with, do probably disagree with his assertion that Christianity mainly attracted mid/upper class citizens and was very quickly made up of a majority of well off, educated converts. Even if mid/upper class citizens were dominantly drawn in, there are a number of problems with Stark’s conclusion. First, literacy was limited to no more than 10% of the population, even upper class citizens were often lacking, so an upper class citizen does not necessarily equal high education – this isn’t explicit in his argument, but it could lead to confusion and we know that early Christians were pretty poor at even copying down their own sacred texts until the fourth century when professional scribes took up the task more often. Second, due to efficient but oppressive Roman imperial policy, there really wasn’t a middle class. 95% of the population was made up of the “peasant” or lower class with no real middle and then a sudden bubble of wealth at the top, especially in the top 1%. Third, authentic writings of Paul (“Not many of you were wise by worldly standards” Paul writes) as well as writings of Origen and Celsus (writing much later than Paul) confirm early Christians were not very well educated and likely were not predominately made up of the mid/upper class.
Stark contends that new religious cults (breakaway religions) tend to draw in more educated, upper class converts while new religious sects (extensions of religions) tend to draw in a broader mix. He then uses this theory to claim Christianity was a middle/upper class movement because immediately after the resurrection it became a Christian cult (even if Paul didn’t think of it this way - what a strange, ignorant thing for Stark to conclude), not the Jewish sect of Jesus. His basis for this, that the resurrection sparked drastically different theological conclusions from Jewish thought, just shows a lack of understanding of resurrection theology and Paul himself as completely based within traditional Jewish thought. Certainly, Jewish Christianity was new, but it was not a cult until much later after Paul had died and Greek thought got the best of the movement.
Despite major Jewish/Christian conflicts arising at least by the writing of Luke in the 70’s (as evidenced by the author’s anti-Jewish re-writings of Markan material continued into Acts), I would contend it wasn’t until near the end of the first century that Christianity became so consciously distinct from Judaism that it became a cult and not just the Messianic Jewish sect of Jesus and Paul (for better or worse, it’s debatable I think). This is in part as it incorporated more Greek thought into the faith than either Paul or Jesus. Paul’s biggest group of converts, “the God-Fearers” (who were the primary people he focused on converting when he went to synagogues - he wasn't called to the Jews but to the Greeks) were probably the earliest converts and need not be associated with upper class citizens. They were truly Greek (avoiding the full implications of the Law), but attracted to Jewish ethics and monotheism – a hybrid that Paul was able to communicate to perfectly. Hellenized Jewish converts probably came later, although they were in a similar religious situation as the God-Fearers. Early Christianity certainly had wealthier members (like Nicodemus) and without them the movement wouldn’t have succeeded (on this much I agree with Stark – the upper class was essential to the success of early Christianity), but they were not the majority in the first century.
Following this is Stark’s lengthy chapter on why the mission to the Jews didn’t actually fail (despite what is claimed in the NT) but continued on in to the fourth century. The cultural continuity Christianity offered to Hellenized Jews was easier to swallow than for the Gentiles who didn’t connect with the Jewish ideas as well. This is key and I think I agree with him for the most part on this point – the rise of Christianity spread quickly to the Hellenized Jews of the Diaspora, who because of Paul’s ability to put Greek clothes on a Jewish message, resonated for obvious reasons. The ethnic requirements of the Law had become a burden in a Hellenistic context for these Jews and they became discontented, another ripe ingredient for conversion to new religious movements. Additionally, the earliest Jewish Christians had pre-existing relationships with these Hellenized Jews – and Stark contends that social movements grow much faster when spreading through existing social networks. The assertion in the NT that the mission to the Jews had failed may have been true at the time if I am correct that the earliest converts came mainly from the peasant class and “the God-Fearers”, and it was only a number of decades after Paul died in the 60’s that the Hellenized Jews began to latch onto Christianity in mass numbers.