John the Baptist

I thought John was a Jew.
A mikveh is a ritual bath for cleansing. I conjecture that the water baptism at the River Jordan was a ritualistic water cleansing.
The principal departure is that Jewish men did the ritual once a year and women after every menstruation. In Christianity, it is accepted that one needed to be baptised only once in your life.

Catholic?

I'd suggest stating with reading History of Baptism in Wikipedia.

The possibility that #2 isn't joking makes my stomach hurt.

John, like his cousin Jesus, was a Jew.

Is Jew a religion? I thought OP was asking for his religion....

#5, I don't know where to file a question like that. While it's obviously true that not all Jews are religious, those who are religiously observant are of the Jewish religion. What a surprise.

#5, I don't know where to file a question like that.

File it under "Egregious Ignorance."

"Jew" is not a religion--it is a proper noun--but Judaism is a religion. Apparently #5 thinks the history of turmoil in the Middle East is about nothing more than the proper way to cook falafel.

Without checking the wikipedia entry, I will add that the cleansing ritual of the mikveh requires flowing water. Mikvehs are often built at natural springs for this reason, and the ritual can be performed in a river, or spring-fed lake, as well.

. . . and John was baptizing in the Jordan, and there are still Christians who baptize in rivers only. The Sabaeans, who claim ideological descent from John, are also very big on flowing water.

(So one answer to OP's question is that the Sabaeans would tell you he was a Sabaean. And I think the Muslims would tell you he was a Muslim, that is, that he accepted the one God, like all true prophets.)