In my previous post I said there were two main movements which developed early on in the first century of the Christian church:
I commented on Gnosticism - the belief that salvation was obtained by understanding certain truths which were hidden from those with a superficial knowledge. A modern counterpart to Gnosticism is that we are saved by having "correct doctrine" and that only those people who believe "the truth" as it is defined by the particular group will be saved. Some Christadelphians come perilously close to being Gnostics in this sense.
The main heresy which Paul resisted was Exclusivism. We encounter this movement in Paul's writings whenever he dealt with the problem of "the Judaisers". While many people think of Paul's writings to be dealing with legalism versus grace, the problem with the Judaisers was not so much one of legalistic observance of law as it was of exclusivism. Several New Testament scholars over recent years have contributed a great deal in this area, including E.P. Sanders, James D.G. Dunn, N.T. Wright, and others. They have shown that Paul's argument with the Judaisers was not about Christian grace versus Jewish legalism but was rather about the status of Gentiles in the church. It was a controversy about whether Gentiles should be accepted into the new covenant people of God as Gentiles or whether they should first convert to Judaism and become Jews. Christianity, after all, was a Jewish religion.
Now, whether or not these new perspectives in contemporary New Testament scholarship on Paul's theology are totally correct, they certainly highlight what was a very real concern of Paul's.* Some members of the church were refusing to acknowledge other converts as genuine believers and required that they do "something more" in order to be accepted. The same doctrine exists today amongst those Christians who refuse to acknowledge other believers as genuine unless they adopt the beliefs and practices of their group or denomination. In it's denominational form it manifests as the view that "only Christadelphians/Jehovah's Witnesses/non-Trinitarians/Trinitarians can be saved". I've seen it go further amongst Christadelphians with the declaration that only members of certain Christadelphian fellowships can be saved! This attitude has produced what we could call "remnant theology" i.e. the view that only "a remnant" will be saved and that most believers are really deluded and only a handful really know "the Truth" which is essential to salvation. (I've actually heard of one Christadelphian fellowship who, when their numbers had dwindled to only 8 people, claimed that they would be the only 8 to be saved "just like Noah's ark"!)
But remnant theology is wrong. God's offer of salvation is inclusive, not exclusive. He is generous with his grace and offers life and life in abundance. Jesus' parables are often about the generosity of God and the lengths to which He will go to secure the salvation of a sinner. In The Revelation John sees a vision of the saved: "a great multitude that no one could count, from every nation, tribe, people and language" (7:9). This is hardly remnant theology.
* For a good summary of whether the issue was exclusivism or legalism in Paul's writings see this article by Mark M. Mattison.