This essay provides fresh insight into the possibility that the passage in Josephus about John the Baptist (Jewish Antiquities, 18.116-119) was not written by Josephus himself. In making the case for its interpolation or adaptation by the hand of a writer representing an early Christian or Jewish-Christian sect, the essay focuses on how the text describes John's baptism and its distinguishing characteristics as well as the similarities it shares with immersions common amid early Christian or Jewish-Christian sects. Of particular importance to uncovering the theological identity of this baptism is its description as an external physical purification, whose efficacy is preconditioned by inner spiritual purification. This essay shows that baptism of this nature did not exist amid mainstream Jewish circles of the Second Temple period. Such baptism appeared and developed within sectarian groups on the margins of Judaism, as at Qumran. It was then carried on and practised by early Christian or Jewish-Christian groups in the first centuries ce.