And the word of God continued to increase, and the number of the disciples multiplied greatly in Jerusalem, and a great many of the priests became obedient to the faith. (Acts 6:7)
“But this I confess to you, that according to the Way, which they call a sect, I worship the God of our fathers, believing everything laid down by the Law and written in the Prophets, having a hope in God, which these men themselves accept, that there will be a resurrection of both the just and the unjust.” (Acts 24:14-15)
“Therefore my judgment is that we should not trouble those of the Gentiles who turn to God, but should write to them to abstain from the things polluted by idols, and from sexual immorality, and from what has been strangled, and from blood. For from ancient generations Moses has had in every city those who proclaim him, for he is read every Sabbath in the synagogues.” (Acts 15:19-21)
It’s interesting to me that, though Gentiles were largely allowed to remain Gentiles when they converted to Christo-Judaism,* Sadducees were expected to become Pharisees when they converted. Of course, it makes sense that Sadducees, who denied an afterlife and a resurrection of the dead, would have to first acknowledge both an afterlife for the soul and the resurrection, since Jesus’ resurrection was a central dogma for Christo-Judaism. But it is also true that the Sadducees represented the traditional approach to the afterlife, the one presented in the Torah, as opposed to the revisionist teaching of the Pharisees, which isn’t found until the post-exilic period, developed in the face of persecution.
Notice the difference between Gentile conversion and Sadducee conversion in the quotes from Acts above. While Luke has James saying that Jews “should not trouble those of the Gentiles who turn to God” (15:19), Luke frames the conversion of Sadducees as “becoming obedient to the faith” (6:7). Likewise, he has Paul saying that the true Christo-Jewish interpretation of “everything laid down by the Law and written in the Prophets” is that “there will be a resurrection of both the just and the unjust” (24:14-15). In other words, while Acts shows the earliest, Pharisaic church being relatively lenient toward Gentile converts, the church in Acts is, to be honest, pretty harsh toward the Sadducees.
Thus, I’d argue that because Sadduceism, to a Pharisee, represented a wrong view of the Scriptures (though the Sadducees actually interpreted the Messianic predictions in the Hebrew Bible more accurately, from the standpoint of reading the Scriptures as the original authors understood them), Sadducees had to become Pharisees in order to “properly” follow the Messiah.
What’s interesting, though, is that since the advent of modern biblical criticism, with its denial of the supernatural, certain groups of Christianity have taken a Sadduceic turn. For this camp, it is no problem if Jesus did not work miracles and did not experience a literal, bodily resurrection. (There is debate over whether Paul and Mark themselves believed in a bodily resurrection, but that’s another post for another day.) The focus in this Christianity thus turns to what Jesus said (his teaching and ethics), rather than what Jesus did (his miracles, his death, and his resurrection).
Personally, I find this kind of Christianity compelling, if not wholly convincing. Jews did not hope for a literal, bodily resurrection until the time of the Maccabees; thus, in order to do responsible exegesis of the OT hope for a Messiah, we cannot include the concept of resurrection from the dead. On this reading, the talk of the supernatural in the New Testament — the afterlife, the resurrection, etc. — is actually a culturally-conditioned addition to the Hebrew Scriptures, and is thus invalid, meaning that our focus should not be a hope for some future bliss after death, but of acting justly and ethically while alive.
* I use “Christo-Judaism” because the term “Jewish Christianity” seems to me to put an undue and anachronistic emphasis on the Christian side of the belief.