Hilary of Poitiers

Hilary is the Athanasius of the West. While Athanasius fought the Arians in Greek in the East, Hilary fought them in Latin in Gaul, and got exiled to Asia Minor for it — which turned out to be a gift, because he learned Greek theology there and brought it home. His De Trinitate was the first serious Latin treatment of the doctrine, and it gave Augustine the categories he later worked with. Without Hilary, Latin Trinitarian theology starts later and weaker. He also taught Martin of Tours, which is how monasticism reached Gaul.