I did a google search on this string - "thomas aquinas faith and reason"
Here is a site that has a short summary at the beginning.
http://www.jcu.edu/philosophy/gensler/re/r-a4--00.htm
One great site is New Advent -
www.newadvent.org
They have this entry under the subsite -
http://www.newadvent.org/cathen/14663b.htm
"(1) Faith and Reason
The principles of St. Thomas on the relations between faith and reason were solemnly proclaimed in the Vatican Council. The second, third, and fourth chapters of the Constitution "Dei Filius" read like pages taken from the works of the Angelic Doctor. First, reason alone is not sufficient to guide men: they need Revelation; we must carefully distinguish the truths known by reason from higher truths (mysteries) known by Revelation. Secondly, reason and Revelation, though distinct, are not opposed to each other. Thirdly, faith preserves reason from error; reason should do service in the cause of faith. Fourthly, this service is rendered in three ways:
•reason should prepare the minds of men to receive the Faith by proving the truths which faith presupposes (praeambula fidei);
•reason should explain and develop the truths of Faith and should propose them in scientific form;
•reason should defend the truths revealed by Almighty God."