It does unfortunately tend to portray those religions as somehow the by-product of Gnosticism, which is not the case.
The purpose of this is to show that, for the countless millions who feel completely disconnected from, say, Islam, that there is in fact an ancient historical relationship not only to the external theologies of Judaism and Islam, or Islam and Christianity, but also a shared, deeper meaning. A familiarity among non-Gnostics with the form and resonance of Gnosticism, I feel, can help in understanding the common ground of all religious experience, particularly among “western” religions. And Gnosticism, as younger than two of these and older than the other two, does seem to serve as a convenient meeting point for a deep-ecumenism.
(via Tim Boucher, who also has some related links).