The slogan "No creed but the Bible" is totally unsustainable in practice. The Bible, after all, is a big book. What does it teach?
A man may accept as the rule of his faith the same inspired books as yourself, while he rejects every important article of the faith you find in these books.Marcus Dods (of all people) quoted by James Bannerman, The Church of Christ Volume 1, p. 298
If, therefore, we are to know who believe as we do, and who dissent from our faith, we must state our creed in language explicitly rejecting such interpretations of Scripture as we deem to be false. Papists, Unitarians, Arminians, all profess to find their doctrines in Scripture; but they do not find them in the Westminster Confession.
No one calling himself a Christian will deny that 'Christ died for our sins;' but out of these words of Scripture a Socinian will bring a meaning which is utterly subversive of what we hold as essential to salvation.
The Church, therefore, gathers her symbol, and utters her Confession, in order that the truth contained in Scripture may be recognised and held in opposition to, or in distinction from, the errors which some have maintained, and which, while they claim to be found in Scripture, are really subversive of the truth therein delivered.