Mythicists (people who claim Jesus is a mythical character) are regularly derided for relying on parallelism for explaining the Jesus story. They compare the Jesus story to myths of other similar pagan gods and find parallels between the tales. Most scholars, who are not mythicists, focus on the Jewish origins of the faith. Yet, the scholars who focus on the Jewish aspects of Early Christianity are also using parallels between Christianity and Judaism in their descriptions. Christianity was not Judaism. The first Christians rejected Judaism, as defined by the Pharisees, the forerunners of Rabbinic Judaism. In the New Testament stories, the Pharisees were the enemies of the Christians. Sure, there are many, many Jewish aspects to Early Christianity; but, Judaism was far from being the only religion in Palestine during the 1st century CE. Judaism gets too much blame for Christianity and those other traditions deserve far more credit in creating the Jesus story. The influence of those other traditions can even be found in the stories about Early Christian characters, commonly accepted as real people.
John the Baptist is usually described as a real, historical person. The Wikipedia article about John even gives him a birthday between 6-2 BCE and death in 36 CE. Baptism and ritual bathing were quite the fashion in several religious traditions of the time; so, there doesn’t seem to be much reason to question the existence of a guy named John, preaching baptism, when the evidence for him is taken at face value. But, the face value description lacks the cultural backdrop needed for understanding his story.
Ritual bathing was a fashion during the Greco-Roman period in the neighborhood of John. Its popularity in Judaism is evidenced by a number of ritual bathing pools found in the region, dating to the Greco-Roman period. But, ritual bathing was also a feature of pagan cults in the Levant. The temple of Eshmun in Sidon was famous for the healing powers of its sacred stream and the priests sold their services as baptizers. Eshmun was one of the several boy-god cults that sprang up during the early Iron Age, as the royal cults of Canaanite/Phoenician city states. All of the boy-gods shared similar myths about a boy who was killed and became the savior god of their respective cities. The boy-gods were local; but, all associated with the great goddess of West Asia, called Dea Syria or Atargatis by the Greco-Roman period. And, she was very popular in all the areas associated with Early Christianity. She was worshipped all along the Jordan River, as evidenced primarily in Nabetaean art. A Hellenistic period Atargatis temple was located in the Gilead, east of the Jordan. She was also worshipped in Gaza and had a temple in Ashkelon. The whole of the Levant was permeated with Dea Syria worship. Her cult sites included ritual fish pools and she was often portrayed as a mermaid during the Greco-Roman period. She became an increasingly fishy character after the 5th century BCE, when the Pisces constellation was redefined. The earlier Babylonian constellation of the goddess as Anunitum with her dove was changed to feature primarily fish in the river of Pisces. Anunitum was one of the many epithets for the goddess best known as Ishtar. Ishtar was a gender bending goddess, represented by the planet, Venus. She changed from female to male, as the morning and evening star. So, she had a male alter-ego.
The origins of ritual fish pools goes back to Mesopotamia, with archaeological evidence of pools decorated with fish and healers portrayed as mermen. Berossos retold myths about the fishy healers in the character of Oannes. Oannes was fish-tailed prophet, who taught alongside waterways and did not eat meat. Oannes showed up in the Levant, where he was conflated with the Canaanite god, Dagan. He also shows up in the Old Testament under a few guises, with Jonah being the most obvious. Oannes and Jonah are the same name and Jonah was a prophet with a fish tale. Oannes is also the same name as John, who taught alongside the river and did not eat meat.
Josephus is credited with providing the proof of a real John because he tells the story of Herod executing John the Baptist. However, it is certain that Josephus was familiar with Berossos’ stories, because Josephus cited Berossos a few times in his books. And, using myths to tell stories about kings was extremely common in ancient history writing. Ancient historians regularly took the “fly on the wall” perspective in telling tall tales about kings. There are many examples of historians using myths as the basis for stories about kings, even about kings who lived near the same time as the historian. So, assuming that Josephus was telling a true story is a very simplistic understanding of ancient history books. There could have been a real person called John executed by Herod. But, the reason he was called John was because of his type of ministry. He was a prophet of John, rather than being the John. It is also possible that the John the Baptist story is yet another example of an ancient historian using myths to make up history. Some critics think that Josephus’ tale about John the Baptist was a later interpolation by a Sabian, which takes the story right back to the Mesopotamian Oannes. No matter how you look at it, the story of John demonstrates the pagan origins of Christianity.