Do you affirm, in doctrine and practice, the 1689 London Baptist Confession of Faith? It is not enough to claim to agree with a statement of faith, we must practice it as well - in other words, the confession must not be some dusty old document referred to in the constitution, but rarely actually referred to by the church leaders and members.
What happened in the mixed denominations in the 20th century was that the confessions were quietly sidelined, and set aside as mere historical documents that had nothing to do with what the Church actually did and believed. This had already happened in the Church of England, where the 39 Articles were generally ignored from the period of the Restoration of the monarchy under Charles II onward. It also happened in the English Prebyterian Churches of the same period, resulting in the practical extintion of that denomination in most of the country. Many Presbyterian Churches became Unitarian, and other Independent. The cause of this was doctrinal laxity, and the lack of confessional subscription. A higher value was set on freedom of conscience than fidelity to God.
As in the 18th century, so in the 20th. C.H. Spurgeon saw the decline hit the Baptists and published the articles on The Down-Grade. He was pilloried for it, but his words proved true. Time and again, when the confessions are abandoned, the faith is not far behind.
Not all matters of present importance are dealt with in the historic confessions, and therefore statements need to be issued on these subjects. But first, let us all look to the Confessions, and affirm them. The problem began, after all, when people stopped referring to the confessions and subscribing them as true summaries of Biblical doctrine.
Today we have two problems that spring from this. The first is apalling doctrinal laxity on confessional issues, and the second is that rather than having statements that can be read, those more conservative churches that claim "no creed but the Bible" have an unwritten confession preserved in tradition which is therefore not available for any prospective member to read. "Back to the Bible" must, practically, also involve going back to written confessions.