Firmilian was bishop of Caesarea Mazaca, an ancient city in Turkey that is now known as Kayseri. Although the dates are not precise, Firmilian served as bishop from AD 230 to his death in 268 or 269. Not much is known of his life and upbringing other than he was probably brought up as a Christian and became a disciple of Origen. What little we know of him is from the historian Eusebius and from a letter Firmilian wrote.
Firmilian of Caesarea is best known for his stance in a controversy over rebaptism. In the Eastern Church, the common practice was to require rebaptism if a person had initially been baptized by someone who was deemed a heretic. In other words, baptism by a false teacher was invalid and needed to be repeated. Likewise, church members who had lapsed and returned to the church were also rebaptized.
In the Western Church, it was not the practice to rebaptize these believers. The reasoning was that, as long as it was a Christian baptism using the Trinitarian formula, the baptism was valid. Any fault in the one performing the baptism did not transfer to the person receiving the baptism in good faith. Likewise, the Western Church did not require rebaptism of lapsed church members.
Firmilian of Caesarea was a strong advocate of rebaptism and opposed Pope Stephen I, who said that rebaptizing people was unnecessary. Firmilian also took issue with Stephen’s insistence upon papal supremacy. Most of what we know about Firmilian’s stance is due to a single surviving letter he wrote to Cyprian on the subject of baptism. (A full English translation of this letter is readily available online.) Either for his theological position or perhaps for his stiff opposition toward Pope Stephen, Firmilian of Caesarea was excommunicated. Later, his excommunication was reversed, and he was restored to communion in the Roman Church. Historian Philip Schaff postulates that Firmilian wrote “with a little more vehemence and acerbity than becomes a bishop” (https://www.ccel.org/ccel/schaff/anf05.iv.iv.lxxiv.html#fnf_iv.iv.lxxiv-p4.3, accessed 8/17/2022).
Today, Firmilian of Caesarea is commemorated in the Eastern Church, where his feast day is October 28. Firmilian is not celebrated in the Roman Catholic Church.