The following are quotes on Porphyry:
“The letter theta (Θ) was, in its archaic form, written as a cross within a circle (⊕, ⊗) and later as a line or point within a circle (Θ, ʘ). According to Porphyry (c.280), the Egyptians used an X within a circle as a symbol of the soul. Having a value of nine ‘9’, it was used as a symbol for the Ennead [nine gods], the nine major deities of the ancient Egyptians. The earliest of these, the great Ennead of Heliopolis, was comprised of the original creator god, Atum, often identified with Ra [Ra-Atum]; his children, Shu and Tefnut; their children, Geb and Nut; and the fourth generation, the brothers, Osiris and Seth, and their sisters, Isis and Nephthys. Johannis Lydus (c.540) noted that the Egyptians also used a symbol in the form of a theta for the cosmos, with an airy fiery circle representing the world, and a snake, spanning the middle, representing the agathos daimon or ‘good spirit’. The Egyptians also used the sign of a point within a circle (ʘ) to represent the sun god Ra, the probable origin of its use as the astrological symbol for the Sun. Coincidentally, theta had the same value in isopsephy as Helios, namely: ΘHTA = 318 = HΛΙΟΣ [Helios]. In classical Athens, theta was also known as the ‘letter of death’ because it was the initial letter of thanatos (death). It survives on potsherds used by Athenians when voting for the death penalty.”
— Kieren Barry (1999), The Greek Qabalah: Alphabetical and Mysticism and Numerology in the Ancient World (pg. 73)