Andrew was born around 870 in Scythian (Slavic Eastern European) territory and taken as a slave to Constantinople, where he served in the household of the Byzantine imperial bodyguard Theognostus.
After Christian formation in the household, he received a vision calling him to the distinctive vocation of the 'Fool for Christ' (Greek Salos, taking up the Pauline-Corinthian phrase 'we are fools for Christ's sake') — abandoning the household to live on the streets of Constantinople, feigning insanity, enduring beating and ridicule, giving away whatever the faithful gave him to the poor, and exercising the hidden charism of spiritual discernment and prophecy.
He is most renowned in the eastern-Catholic Byzantine-rite and Orthodox tradition for the vision at the Blachernae Church on a night in the early 10th century — when the city of Constantinople was besieged by Saracen / Slavic enemies (variously identified): Andrew and his disciple Epiphanius saw the Mother of God herself standing above the congregation in the Blachernae Church, spreading her maphorion (the outer veil) over the worshipping faithful as a sign of her maternal protection of the city.
The vision became the origin of the eastern-Catholic Byzantine-rite and Orthodox Feast of the Protection of the Theotokos (Pokrov / Pokrov Bogoroditsy) — established in the Russian / Slavic Orthodox-Catholic calendar by Prince Andrew Bogolyubsky of Vladimir in 1164 as a national feast (October 1), and one of the principal Marian feasts of the Slavic Catholic-Orthodox liturgical year to the present day.
Andrew died on the streets of Constantinople around 936; his disciple Nicephorus the priest of Hagia Sophia composed his Greek Vita (preserved in the Byzantine hagiographical corpus), which became the primary literary template of the entire Byzantine-Russian holy-fool (yurodivy) tradition that would produce Sts. Basil Fool-for-Christ of Moscow (16th c.), Xenia of St. Petersburg (18th c.), and the continuing Catholic-Orthodox 'fool for Christ' charism.