A few years ago I heard of the Mandaeans for the first time. I'm interested in them because John the Baptist is considered to be one of their prophets. While most scholars have ignored their traditions as having anything to say about early Judaism and/or early Christianity, this may have been an oversight. Prof. James McGrath (Butler University) has given a lecture on some of his recent research on this group that may interest others, which I'm embedding below. Pay attention to how Jerusalem, John, Jesus, the Holy Spirit, and the event of Jesus' baptism are portrayed in this tradition if you watch the lecture.
Source: James McGrath's blog Exploring Our Matrix, "The First Baptists, the Last Gnostics"