> "[The Marcionites] were perhaps the most dangerous foe Christianity has ever known"
Catholic encyclopedia on "Marcionites": https://www.newadvent.org/cathen/09645c.htm
Stumbled upon this from people posting about other heresies and was thinking more reading about heresies might be helpful for people to understand and avoid error
But there are so many different modern and historical interpretations of this stuff that Christianity necessarily has some agnosticism baked into it for anyone who engages seriously with the whole library. Trying to figure out the single, narrow, exclusively "correct" school of theology is like going on fanfiction.net, reading the entire body of work for a popular IP, and then having to independently decide which author has "the canon" interpretation of the source material. All with you never having accessed that source material yourself nor having any way to do so.