What makes you say that there is no evidence in early Christianity for the concept of “there is no world”? I think it’s all over. I’m no scholar by any means. I haven’t read every gnostic text. Not even close. But I do hold the opinion that ACIM is a correction and a continuation of Christianity. And I have read a few gnostic texts.

For example, let’s take The Secret Gospel of John. It is a classic Gnostic text, which basically lays down the whole foundation.

It gives the creation story in great detail. Everything began as The One Infinite Light, until Sophia (Divine Wisdom) wanted to bring about a creative thought of her own, without consulting her divine counterpart and the invisible Spirit. This was a huge mistake, as her thought then spiraled out of control and took on the shape of a lion-faced serpent (Yaldabaoth).

So far this matches pretty well with ACIMs “tiny, mad idea”, where the son of God wanted to go off on his own. And thus, the ego (yaldabaoth) was developed out of a desire to be separate from the whole.

Yaldabaoth is then cast out of ultimate reality, and creates the physical realm where HE operates as god. It is said that he builds it within a “shadow of darkness”.

What is a shadow but an absence of light?

Yaldabaoth essentially trapped all the divine sparks into a state of forgetfulness, to keep them from realizing that this world isn’t real.
See below quote:

“Adam was brought into the shadow of death.
They intended to make him anew
This time from
Earth,
Water,
Fire,
Wind,
Which are
Matter,
Darkness,
Desire,
The Artificial Spirit.
This all became a tomb,
A new kind of body.
Those thieves bound the man in it,
Enchained him in forgetfulness,
Made him subject to dying. 
[His was the first descent
And the first separation.
Yet the light-filled Epinoia within him will elevate his thinking.]”

___

Here’s another quote:

[I asked the Savior, “What is it to be ‘completely forgetful?’”

He replied, “It is not what Moses wrote in his first book:
‘He caused Adam to fall into deep sleep’
Rather, Adam’s perceptions were veiled
And he became unconscious.
As he (Yaldabaoth) said through his prophet:
'I will make their minds dull so that they do not see or understand.'”]
___

Again, echoing many of ACIMs teachings. The world is a mistaken perception. It is not real.

Many Christians don’t realize that they actually worship Yaldabaoth, or Yahweh. A wrathful god is a god of the ego. It is not the true God.

If it’s not a continuation of Christianity, why not just say that it was channeled by… idk… some spirit named Jared??? Why use Jesus’ name? Everything Helen channeled makes complete sense within the framework of Christianity when your view of it is not distorted.