Firmilian of Caesarea, Letter to Cyprian 19
But regarding the custom they appear to set against the truth, who is so foolish as to prefer custom to the truth, or who, having seen the light, would not abandon the darkness? Unless the Jews, with Christ coming — that is, with the truth — are helped in some way by their most ancient custom, since they abandoned the new way of truth and remained in antiquity. Indeed, you Africans can say this against Stephanus: having known the truth, you abandoned the error of custom. As for us, we join both truth and custom, and the custom of the Romans to custom, but we oppose custom to truth, holding from the beginning what was handed down by Christ and the Apostles. Nor do we recall this ever having begun among us, since it has always been observed among you that we acknowledge only one Church of God, and we regard holy Baptism as belonging only to the holy Church. Clearly, since some were doubting the baptism of those who, though they do not receive the Prophets as we do, nevertheless seem to know the same Father and Son as we do, many of us gathered together in Iconium, carefully discussed the matter, and confirmed that every baptism established outside the Church must be repudiated.
Latin
Quod autem pertinet ad consuetudinem refutandam, quam videntur opponere veritati, quis tam vanus sit ut veritati consuetudinem praeferat, aut qui, perspecta luce, tenebras non derelinquat? Nisi si et Judaeos, Christo adventante, id est veritate, adjuvat in aliquo antiquissima consuetudo, quod, relicta nova veritatis via, in vetustate permanserint. Quod quidem adversus Stephanum vos dicere, Afri, potestis, cognita veritate, errorem vos consuetudinis reliquisse. Caeterum nos veritati et consuetudinem jungimus, et consuetudini Romanorum consuetudinem, sed veritatis opponimus, ab initio hoc tenentes quod a Christo et ab Apostolis traditum est. Nec meminimus hoc apud nos aliquando coepisse, cum semper istic observatum sit ut nonnisi unam Dei Ecclesiam nocemus, et sanctum Baptisma nonnisi sanctae Ecclesiae computaremus. Plane, quoniam quidam de eorum baptismo dubitabant qui, etsi non ut nos Prophetas recipiunt, eosdem tamen Patrem et Filium nosse nobiscum videntur, plurimi simul convenientes in Iconio, diligentissime tractavimus et confirmavimus repudiandum esse omne omnino baptisma quod sit extra Ecclesiam constitutum.