structural:summarization
Mystery religion initiation rites

Many scholars have drawn parallels between rites from mystery religions and baptism in Christianity. Apuleius, a 2nd-century Roman writer, described an initiation into the mysteries of Isis. The initiation was preceded by a normal bathing in the public baths and a ceremonial sprinkling by the priest of Isis, after which the candidate was given secret instructions in the temple of the goddess. The candidate then fasted for ten days from meat and wine, after which he was dressed in linen and led at night into the innermost part of the sanctuary, where the actual initiation took place, the details of which were secret. On the next two days, dressed in the robes of his consecration, he participated in feasting. Apuleius describes also an initiation into the cult of Osiris and yet a third initiation, of the same pattern as the initiation into the cult of Isis, without mention of a preliminary bathing. The water-less initiations of Lucius, the character in Apuleius's story who had been turned into an ass and changed back by Isis into human form, into the successive degrees of the rites of the goddess was accomplished only after a significant period of study to demonstrate his loyalty and trustworthiness, akin to catechumenal practices preceding baptism in Christianity.

Jan Bremmer has written on the putative connection between rites from mystery religions and baptism:

There are thus some verbal parallels between early Christianity and the Mysteries, but the situation is rather different as regards early Christian ritual practice. Much ink was spilled around 1900 arguing that the rituals of baptism and of the Last Supper derived from the ancient Mysteries, but Nock and others after him have easily shown that these attempts grossly misinterpreted the sources. Baptism is clearly rooted in Jewish purificatory rituals, and cult meals are so widespread in antiquity that any specific derivation is arbitrary. It is truly surprising to see how long the attempts to find some pagan background to these two Christian sacraments have persevered. Secularising ideologies clearly played an important part in these interpretations but, nevertheless, they have helped to clarify the relations between nascent Christianity and its surroundings.

Thus the practice is derivative, whether from Judaism, the Mysteries or a combination (see the reference to Hellenistic Judaism in the Etymology section.)