The Origin of Christianity's Sectarianism
The pattern of Jewish religion called "covenantal nomism", coined and described by E.P. Sanders, declares that physical descent from Abraham and Torah-obedience guarantees one entry into God's kingdom. First-century Judaism was largely about covenant religion, and Sanders believes that Jesus' thinking squared with it, that it was Paul who later shot down the twin pillars of covenantal nomism -- election and the law.
Dale Allison has critized Sanders on this point in "Jesus and the Covenant: A Response to E.P. Sanders", JSNT 29 (1987), pp 57-78, arguing that Jesus, like his mentor the Baptist, completely rejected covenantal hope. The two prophets believed that one must be "born again", that deliverance came not by belonging to the Jewish people or having the law, but through a radical turning around -- a radical repentance producing good fruit. Indeed, says Allison, the entire New Testament tradition leaves the question of one's salvation open: people should be rather worried about their fate in the kingdom.
Joan Taylor likewise thinks the Baptist started this understanding:
"[According to the Baptist] God can, if he so wishes, make new children of Abraham out of stones lying around on the ground, just as he created the first human being out of the dust of the earth. One cannot look to a community that followed God's Law and expect inherited zekhut; it has to be earned...in the present time by each person in his or her own life; only then can s/he truly continue the spirit of Abraham." (The Immerser, pp 129-130)
Sounds a lot like Paul in Rom 9-10. I think Allison and Taylor are entirely correct. Christianity's sectarian rejection of covenantal nomism goes right back to the point of origin, to the Baptist and Nazarene themselves.
UPDATE: It should be noted that despite the above, Taylor insists that neither John nor Jesus were sectarian, in the sense that "nothing about John's message requires us to assume that he intended to immerse people to form an exclusive group that might deem themselves to be God's faithful remnant in the last days...there is nothing sectarian or exclusive here." (The Immerser, pp 148-149). I have doubts about this, but am using the term sectarian more generally in any case: John and Jesus rejected common covenantal hopes and salvific avenues. I believe L. Michael White's use of the term applies to both John and Jesus: "The Jesus movement is a sect... A sect always arises within a community with whom it shares a basic set of beliefs and yet, it needs to find some mechanism for differentiating itself. So sectarian groups are always in tension with their environment. That tension is manifested in a variety of ways -- controversies over belief and practice; different ideas of purity and piety. But another manifestation of that tension is the tendency to want to spread the message out, to hit the road and convince others that the truth is real."