From Captive Prince to Monk
The earliest recorded fact of Serapion's life places him among war captives. In 1551 the Kazan Tatars brought captives to Moscow, and among them, by the synaxarion's account, was the myrza (Tatar prince) Turtas Gravirovich. He was baptized with the name Sergius and entered the household of the Moscow boyar Zachariah Plescheev.
The sources stress the depth of his conversion: Sergius embraced the Christian faith so sincerely that he decided to dedicate himself wholly to God. The OCA troparion for his feast frames this turn in stark terms, describing him as one who rejected what it calls 'the Hagarene wickedness' of his family and abandoned worldly honors to take up the monastic and eremitic life in the wilderness.