Throughout the history of the church there have been groups which have argued that "it can't really be that simple!" and have found creative ways of making the Gospel more complicated, sometimes completely obscuring the original message in the process.
It's a trend which began in the first century, even before the New Testament was completed. There were two main movements which developed early on:
I'd like to comment first on Gnosticism. The underlying ideologies in Gnosticism began even before Christianity and may have entered the church through Jewish Gnostics who converted to Christianity. The word gnostic comes from the Greek gnosis "to know" and the Gnostics were thus named because they believed all was not what it appeared to be on the surface and in order to be saved you had to have a special "knowledge" of the "hidden truths". John's writings appear to have been written to counter this directly, and he encourages his readers with the reassurance that they do indeed know the truth, and that yes, it really is very simple! It's possible that in some of Paul's writings he dealt with Gnosticism as a secondary problem, although his main concern was exclusivism, which I'll comment on in a later post.
Now, while I'm not suggesting that Christadelphianism has adopted any of the uniquely Gnostic doctrines, I am concerned about the emphasis (especially in some Christadelphian circles) on the need for knowledge and correct doctrine as being "essential for salvation". This actually drives some people away from Christ because they may feel inadequate about their academic or intellectual ability to grasp all the fine details. I've met a lot of young people who have delayed baptism (some never getting baptised at all) because they "don't know enough". While Jesus demanded a decision from his listeners and called them to repentance stressing the urgency of a response, Christadelphians typically have a long learning process before anyone can be baptised. The process can take years, and some people have even been denied baptism because they were confused, uncertain, or lacked knowledge about fine details.
No wonder that both John and Paul resisted the trend towards gaining knowledge as a means of salvation.