It was the dual symbol of Heaven and Earth, of Sun and Moon, and was made sacred, in consequence of its amphibious nature, to Osiris and Isis. According to Eusebius, the Egyptians represented the sun in a ship as its pilot, this ship being carried along by a crocodile "to show the motion of the Sun in the moyst (Space)", (Prepar. Evang., 1, 3, c. 3). The crocodile was moreover, the symbol of Egypt herself -- the lower, as being the more swampy of the two countries. The Alchemists claim another interpretation. They say that the symbol of the sun in the ship on the Ether of Space meant that the hermetic matter is the principle, or basis, of gold, or again the philosophical sun; the water, within which the crocodile is swimming, is that water or matter made liquid; the ship herself, finally, representing the vessel of nature, in which the sun, or the sulphuric, igneous principle, acts as a pilot: because it is the sun which conducts the work by his action upon the moist or mercury.
The above is only for the Alchemists.
The Serpent became the type and symbol of evil, and of the Devil, only during the middle ages. The early Christians -- besides the Ophite Gnostics -- had their dual Logos: the Good and the Bad Serpent, the Agathodaemon and the Kakodaemon. This is demonstrated by the writings of Marcus, Valentinus, and many others, and especially in Pistis Sophia -- certainly a document of the earliest centuries of Christianity. On the marble sarcophagus of a tomb, discovered in 1852 near the Porta Pia, one sees the scene of the adoration of the Magi, "or else," remarks the late C. W. King in "The Gnostics," "the prototype of that scene, the 'Birth of the New Sun.'" The mosaic floor exhibited a curious design which might have represented either (a) Isis suckling the babe Harpocrates, or (b) the Madonna nursing the infant Jesus. In the smaller sarcophagi that surrounded the larger one, eleven leaden plates rolled like scrolls were found, three of which have been deciphered. The contents of these ought to be regarded as final proof of a much-vexed question, for they show that either the early Christians, up to the VIth Century, were bond fide pagans, or that dogmatic Christianity was borrowed wholesale, and passed in full into the Christian Church -- Sun, Tree, Serpent, Crocodile and all.
"On the first is seen Anubis . . . holding out a scroll; at his feet are two female busts; below all are two serpents entwined . . . a corpse swathed up like a mummy. In the second scroll . . . is Anubis, holding out a cross, the "Sign of Life." Under his feet lies the corpse encircled in the numerous folds of a huge serpent, the Agathodaemon, guardian of the deceased. . . . . In the third scroll, Anubis bears on his arm . . . . . the outline of . . a complete Latin cross . . . At the god's foot is a rhomboid, the Egyptian 'Egg of the World,' towards which crawls a serpent coiled into a circle . . . . Under the busts is the letter [[omega]] repeated seven times in a line, reminding one of the 'names' . . . Very remarkable also is the line of characters, apparently Palmyrene, upon the legs of the first Anubis. As for the figure of the serpent, supposing these talismans to emanate not from the Isiac but the newer Ophite creed, it may well stand for that "True and perfect Serpent," who leads forth the souls of all that put their trust in him out of the Egypt of the body, and through the Red Sea of Death into the Land of Promise, saving them on their way from the Serpents of the Wilderness, that is, from the Rulers of the Stars." (King's "Gnostics," p. 366.)
And this "True and Perfect Serpent" is the seven-lettered God who is now credited with being Jehovah, and Jesus One with him. To this Seven-vowelled god the candidate for initiation is sent by Christos, in the Pistis Sophia, a work earlier than St. John's Revelation, and evidently of the same school. "The (Serpent of the) Seven Thunders uttered these seven vowels," but "Seal up those things which the seven thunders uttered, and write them not," says Revelation. "Do ye seek after these mysteries?" inquiries Jesus in Pistis Sophia. "No mystery is more excellent than they (the seven vowels): for they shall bring your souls unto the Light of Lights" -- i.e., true Wisdom. "Nothing, therefore, is more excellent than the mysteries which ye seek after, saving only the mystery of the Seven Vowels and their FORTY AND NINE Powers, and the numbers thereof."
In India, it was the mystery of the Seven FIRES and their forty-nine fires or aspects, or "the members thereof," just the same.
These seven vowels are represented by the Swastika signs on the crowns of the seven heads of the Serpent of Eternity, in India, among esoteric Buddhists, in Egypt, in Chaldea, etc. etc., and among the Initiates of every other country. It is on the Seven zones of post mortem ascent, in the Hermetic writings, that the "mortal" leaves, on each, one of his "Souls" (or Principles); until arrived on the plane above all zones he remains as the great Formless Serpent of absolute wisdom -- or the Deity itself. The seven-headed serpent has more than one signification in the Arcane teachings. It is the seven-headed Draco, each of whose heads is a star of the Lesser Bear; but it was also, and pre-eminently, the Serpent of Darkness (i.e., inconceivable and incomprehensible) whose seven heads were the seven Logoi, the reflections of the one and first manifested Light -- the universal LOGOS.