"How can you be sure that any particular confession of faith has it 100 per cent right?"
By the same token how can one be sure his own bible understanding is always correct, or his pastor's? If your Pastor errs then why do the congregation usually follow- despite believing they are bible bereans? Think of 'the brethren' for example-no confesssion-yet often much error despite the claim that they have ONLY the Bible. A confession is in black and white and can be accepted or rejected-nothing hidden or mysterious. At least you know what the church believes on doctrine and what your pastor is stating he believes BEFORE he enters the pulpit. It was a certain Mr Russell who rejected the Westminster confession and founded the WT Society. Nothing wrong with that confession particularly on the trinity! How long before you spot an anti-trinitarian if he doesn't expound the word of God and simply gives a light gospel message about faith in the son of God (like many evangelicals)? Like 'oneness pentecostals'-it might take a while!!