What did the Early Christians Believe?
Baptism: Baptism by Blood (Martyrdom) (part1)
"We have, indeed, a second [baptismal] font which is one with the former [water baptism]: namely, that of blood, of which the Lord says: ‘I am to be baptized with a baptism’ [Luke 12:50], when he had already been baptized. He had come through water and blood, as John wrote [1 John 5:6], so that he might be baptized with water and glorified with blood. . . . This is the baptism which replaces that of the fountain, when it has not been received, and restores it when it has been lost" (Tertullian, Baptism, 16 [A.D. 203]).
"[Catechumens who suffer martyrdom] are not deprived of the sacrament of baptism. Rather, they are baptized with the most glorious and greatest baptism of blood, concerning which the Lord said that he had another baptism with which he himself was to be baptized [Luke 12:50]" (Cyprian of Carthage, Letters, 72:22 [A.D. 253)