A couple of points:
Firstly, if you join a church that 'just believes the Bible,' you have no idea what it believes the Bible teaches, and no defence against it changing its mind. I do not foresee myself leaving my current church, but if for some reason I did, I would never join a church without a statement of faith that covered more than the basics because firstly I wouldn't know what I was getting into, and secondly it would be open to infiltration by people with other agendas.
The first Protestant confessions were written to show just where the new churches differed from the Roman Catholic church. The 1644 Baptist confession was designed to show, in a time of persecution, that the Baptist churches were orthodox in their beliefs, and not renegade like some of the Anabaptists. The 1689 Confession (actually produced in 1677) was also introduced at a time of persecution against the free churches, and was brought in to show the level of agreement between the Particular Baptists and the Presbyterians (Westminster Confession) and the Congregationalists (Savoy Confession). those three documents are similar in many places and helped to draw the non-conformist churches together at a very difficult time.
After the Toleration Act of 1689 which gave the Free churches the right to worship unhindered in their own buildings, many churches abandoned their confessions and this led to a period of great declension and the rise of Unitarianism. Those who are interested may read about it here:
Learning the Lessons of History The Salters’ Hall Synod of 1719 Henry Ford was quite forthright. “History,” he said, “is more or less bunk.” The Roman statesman Cicero had a very different view. “H…
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[Read first I am sure that I cannot be alone in seeing remarkable parallels between the early 18th Century and today. In our own time we have seen a marked decline in Christian orthodoxy and teac…
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The decline of orthodoxy in PCUSA, for example, can be traced to the abandonment of the WCF. In my view, statements of confessions of faith are vital to the health of Bible-believing churches.