He was the author of the Athanasian Creed (Link), a basic statement of the Catholic faith (as he says in its first line)(tip'o'the hat to the Orthodox as well); he defended the Incarnation and the Trinity; thus he was a contributor to the Sacred Tradition.
Very much so. His creed is the most definitive Christian Creed. And, as you have very well pointed out, he was a most orthodox Catholic and promulgated the Faith very very well.
:o)