I have read some of St. Thomas's Summa Theologica, and I am confused. Question: In his writings does he support the filioque addition?
Thomas Aquinas was one of several theologians who did not accept the concept of the Immaculate Conception, as it was not considered to be Catholic dogma then.
However, since he was born after the Great Schism (1054 A.D.) and the filioque was one of the main reasons for the schism between Catholics and Orthodoxy, yes, he supported the doctrine of the filioque. Nevertheless, he was on his way to the Council of Florence to help "solve" the schism when he died.