The First Confession of Basel (1534) says (the text I am citing here comes from James T. Dennison, Jr., ed., Reformed Confessions of the 16th and 17th Centuries in English Translation, Vol. 1 (1523-1552), pp. 295-296:
Of the Finishing Touch to the Confession of Basel, which has been Demonstrated from Sacred Scripture Alone, as it were a Singular and Perfect Rule of Faith, we Submit [this] for Evaluation with a Promise of Emendation if any Place in an Article is Against what is Written [in Scripture]
Finally, we submit our confession to the judgment of sacred Scripture: and promise that if we are instructed in better things from the Scriptures as preached, we always intend to obey God and His most holy Word with great thanksgiving.
Ward cites also a note appended to the First Helvetic Confession (1536) which reads (see Philip Schaff, Creeds of Christendom, 1:389, for further discussion):
A declaration of witnessing of our mind. It is not our mind for to prescribe by these chapters a certain rule of the faith to all churches and congregations, for we know no other rule of faith but the holy scripture and therefore we are well contented with them that agreeth with these things, howbeit they be in another manner of speaking or confession different apartly to this ours in words, for rather should the matter be considered than the words. And therefore we make it free for all men to use their own sort of speaking as they shall perceive most profitable for their churches, and we shall use the same liberty. And if any man shall attempt to corrupt the true meaning of this our confession he shall hear both a confession and a defence of the verity and truth. It was our pleasure to use these words at this present time that we might declare our opinion in our religion and worshipping of God.
Next he cites the preface to the 1560 Scots Confession:
[I]f any man will note in our Confession any chapter or sentence contrary to God's Holy Word, that it would please him of his gentleness and for Christian charity's sake to inform us of it in writing; and we, upon our honour, do promise him that by God's grace we shall give him satisfaction from the mouth of God, that is, from Holy Scripture, or else we shall alter whatever he can prove to be wrong.
The theme of all historic Reformed confessions and creeds and catechisms is that they aim to teach only that which is agreeable to the Word of God, and not to perpetuate the opinions and traditions of man unsubstantiated by God's Word. The only rule of faith is the Word of God, and creedal statements are subordinate to that standard. Their authority is derivative from the true source of authority, God's Word, but insofar as it is agreeable to the same, it is authoritative indeed.