Here's more from Aquinas on sola scriptura, courtesy of Joel in Georgia (thanks Joel):
"Objection: It would seem that it is unsuitable for the articles of faith to be embodied in a creed. Because Holy Writ is the rule of faith, to which no addition or subtraction can lawfully be made, since it is written (Deut. 4:2): “You shall not add to the word that I speak to you, neither shall you take away from it.” Therefore it was unlawful to make a creed as a rule of faith, after Holy Writ had once been published.
Reply: The truth of faith is contained in Holy Writ, diffusely, under various modes of expression, and sometimes obscurely, so that, in order to gather the truth of faith from Holy Writ, one needs long study and practice, which are unattainable by all those who require to know the truth of faith, many of whom have no time for study, being busy with other affairs. And so it was necessary to gather together a clear summary from the sayings of Holy Writ, to be proposed to the belief of all. This indeed was no addition to Holy Writ, but something gathered from it.
(St. Thomas Aquinas c. 1224-1274, Summa Theologica, Second Part of the Second Part, Question 1, Article 9)"
Aquinas is here answering the standard objection to creeds (even in his day! i.e., that they are additions to scripture). Note that he works from the assumption of sola scriptura, i.e. that scripture alone is the rule of faith, which can neither be added to nor subtracted from.