I’ve looked at this symbol before, but in relation to Sharon Tate’s friend, Jay Sebring’s use of it in his advertising. But I haven’t explained its significance to me, personally. Perhaps this chapter from Jesus and the Lost Goddess will help:
The Christian myth cycle suggests that the original source of the Christian image of the cross is the ubiquitous Egyptian cross the ankh. This ancient symbol was so important to Christians that they tooled it into the leather cover of one of the collections of gospels found at Nag Hammadi. The basic ankh is a circle above a ‘T’ cross. The crosses that were used for crucifixions were ‘T’-shaped and in Jewish mythology Moses lifts up a serpent on a ‘T’ cross in the desert, which was understood as a pre-echo of the image of Jesus on the cross. For these reasons, the early Christians thought of the Christian cross as ‘T’-shaped. The ankh, therefore, would have been seen as the cross of Jesus below a circle. The circle is a symbol of wholeness and nothingness. It represents the archetypal potentiality of the pleroma. The ankh shows the pleroma as delineated by the horizontal axis of the cross, just as it is in Christian mythology. Below the circle, the kenoma is cut in two by the descending vertical axis of the cross, representing the Mystery made manifest through duality. The ankh is expressing the idea of syzergy — the One as two.
Mirrors were constructed in the shape of an ankh, with the cross as the handle and the circle as the mirror. The circle and the cross represent that which is whole and that which is divided — the essence and the appearances, the Mystery and the manifest, the pleroma and the kenoma. Understanding the relationship between the poles of this fundamental duality is the key to the realization of Gnosis. And what is that relationship? It is one of reflection. The image is a reflection of the archetype. The appearances are how the essence looks to itself. Wanting self-knowledge, the One becomes two. The Mystery becomes the viewer and the mirror. It creates a concept of what it is. It reflects on itself. From these two — thinker and thought — comes the cosmos.Jesus and the Lost Goddess (pages 158-9)