Achillas was the 18th Patriarch of Alexandria, reigning from 312 to 313.
He was born in Alexandria, Egypt, and was renowned for his knowledge and piety; this was why Pope Theonas had ordained him priest and appointed him head of the Catechetical School of Alexandria upon the departure of Pierius. He was apparently very highly thought of for his work in Greek philosophy and theological science, as Pope Athanasius later described him by the honorific "Achillas the Great".
Saint Mark's Coptic Orthodox Cathedral is a Coptic church in Alexandria, Egypt. It is the historical seat of the Pope of Alexandria, the head of the Coptic Orthodox Church.
Believed to stand on the site of a church founded in AD 42 by Mark the Evangelist, in AD 311 a chapel was recorded here, containing bodies said to be of Mark and his successors. Enlarged in the days of Pope Achillas, the church was ruined in 641 when the Arabs invaded Egypt, but was rebuilt in 680 by Pope John III of Alexandria. In 828, most of the body of Saint Mark was stolen and removed to Venice, with only the head remaining in Alexandria.