Episcopate and the Council of Nicaea
According to tradition, Achilles was distinguished for his holiness of life and his learning before he became bishop of Larissa in Thessaly. He served during the reign of Saint Constantine the Great.
He took part in the First Ecumenical Council, where the assembled fathers addressed the Arian heresy. The synaxarion records that he boldly denounced the heresy, and he became renowned for his vigorous defense of Christian orthodoxy. The OCA troparion (Tone 4) describes him as 'a rule of faith, an image of humility and a teacher of abstinence.'
By tradition, after returning from Nicaea he demolished many pagan temples, built numerous churches, and performed exorcisms throughout his diocese. The 1910 Catholic Encyclopedia noted him as a bishop celebrated for his miracles.