Anthony was born on the Grand Duchy of Lithuania territory into an Orthodox family, although his social status and lay name remain unknown.
Considering his penance insufficient, Anthony asked the abbot for permission to go to a Muslim country, where he might receive martyrdom, which was then refused.
Anthony only received permission to go to Mount Athos, where he vowed Great Schema and took the monastic name Onuphrius.
He then went to Thessalonica, into the Church of the Theotokos Acheiropoietos, which had been converted into a mosque, and began to pray demonstratively as a Christian.
As was the custom in the Ottoman Empire at the time, the kadi offered to pardon him in return for his conversion to Islam, which he consistently refused while attacking the religion.