Have you ever actually read the original writings of theosophists and gnostics? They talk about how everything is a manifestation of God, how there is only one God, etc. How can that be different from the monotheistic god of any of the religions?

Yes, and the Devil is actually the ancient sumerian Ba'al Principality, the demon god of 1000 faces.

All ancient Pagan gods are the fallen ones aka demons, that the New Age Theosophists worship.

The Esoteric Meaning of Lucifer

Alice and Foster Bailey were serious students and teachers of Theosophy, a spiritual tradition which views Lucifer as one of the solar Angels, those advanced Beings Who Theosophy says descended (thus “the fall”) from Venus to our planet eons ago to bring the principle of mind to what was then animal-man. In the theosophical perspective, the descent of these solar Angels was not a fall into sin or disgrace but rather an act of great sacrifice, as is suggested in the name “Lucifer” which means light-bearer.

Source: https://www.lucistrust.org/arcane_school/talks_and_articles/the_esoteric_meaning_lucifer

In gnosticism there is this idea of a "demiurge" yaldabaoth which created our physical universe to use the beings within as slaves.

The fake ass "demiurge" creator entity is actually identified as Saturn/Satan, and his abomination of a mother Sophia is identified as Venus/Lucifer.

Luciferianism, the New Age Movement - Russ Dizdar (2011) https://rumble.com/v335tw8-russ-dizdar-luciferianism-the-new-age-movement-audio.html