Achillas had been ordained a presbyter by Pope Theonas of Alexandria with Pierius, and was made the head of the Catechetical School of Alexandria upon the departure of Pierius, who had either left for Rome or been made a martyr at Alexandria. He was apparently very highly thought of for his work in Greek philosophy and theological science, as Athanasius of Alexandria later described him by the honorific "Achillas the Great". He is commemorated in the Synaxarion of the Coptic Orthodox Church on 19 Ba'ūnah.