What precisely must one believe to be saved? Given the variety of people’s background, understanding and what is told them of Christianity, even as exampled in Scripture, that is not an easy question to answer.
If people profess serious errors that seem to overturn the fundamentals of Christianity, though they yet profess to believe in those fundamental teachings of the True Faith, will they be saved? Are they professing Christians? May we have fellowship with such a Church?
The topic of the fundamental and secondary teachings of Christianity is at once a profoundly basic issue, and yet it is incredibly complex with many real world implications. Should a baptist be allowed to be a member of a church reformed according to the Word of God? Should an Arminian?
Little is it known that there was a large, reformed consensus in these issues amongst the presbyterians in the puritan era, even in the minute details, and that not simply from human agreement, but as it was proved by the Word of God and the light of nature.
You will learn a lot. See especially Witsius. And, as usual, git wrekt by Rutherford: