One of the biggest threats to what would become orthodox Christianity was Gnosticism. There had been other theological innovations in the early church but Gnosticism was the most radical. And the most confusing.
A helpful book for moderns readers to understand this complex movement is The Gnostics: Identifying and Early Christian Cult by Alastair H. B. Logan.
Logan demonstrates that there was not just one group known as the Gnostics. There were various groups, the two Logan mostly focuses on are Valentinians and Sethians, but there were more.
Logan takes on a journey looking at the writings of the Gnostics that we have, thanks to the Nag Hammadi Library, as well as the early Christian apologists.
Logan also looks at the various connections to Greek philosophy, especially various strains of Platonism.
This is a scholarly work but one that is accessible to a larger audience. It has been a helpful resource in my current research on Gnosticism.
