The Patriarch Paul II sent him on a mission to Constantinople, the capital of the Byzantine Empire, where he was detained on the orders of the Emperor Justinian I.
[1] On his deathbed,[2] Patriarch Theodosius I of Alexandria commissioned Longinus to continue the evangelisation of the Nubian kingdom of Nobadia that had begun under Julian the Evangelist and Theodore of Philae in 543, but had been interrupted in 551.
[1] He built the first church in Nubia, probably the mudbrick building which was discovered beneath the ruins of Faras Cathedral.
[3] In 575, Longinus was consulted by Syrian envoys concerning the readmission of the deposed Patriarch Paul II into communion.
[4] That same year, Longinus returned to Alexandria because Theodosius had died and the patriarchal office was vacant.
[1][2] Accompanied by a Nobadian royal escort,[2] he took the Korosko Road[3] through the Eastern Desert in a Blemmyan camel caravan.