Satan, is derived from the hebrew "ha sheth'n," meaning "the adversary," I believe. The word satan is derived from the same root as the word "seteh," which means "to turn away." It is notable that seteh is a variant pronunciation of the Egyptian god Set (also Seteh. Sutekh, Setesh, Seth).
The location of Set worship being near the end of the Nile combined with these lexical similarities seem to likely be more than a coincidence. Also, the use of names of other deities to refer to what later becomes the new testament concept of "Satan," is quite present in the Old Testament. Examples include Beelzebub-a deity worshipped in Ekron, Levithian-a jewish mythological sea monster, Baal-though mainly used as a title it can refer to a number of deities worshiped around Carthage. This is continued in the New Testament with Lucifer, a latin translation of the greek (H)Eosphoros, or "Morning Star." This deity was likely derived from the Assyrian Ishtar or the Egyptian Astarte (a wife of Set).
The most interesting of these is Belial (also Baalat, Baalitor, Belit ili). This term is likely a perversion of the Babylonian goddess Belili, which was converted it to the hebrew Belial, a combination of the words "beli" and "yo'il" which together mean "worthless." This same goddess may have also served to be the basis for the pre-Eve figure of Lilith from jewish mythology.
Here it is important to indicate that the concept of "Satan" in the hebrew tradition differs greatly from the christian concept of "Satan". The phrase "ha sheth'n" is used to refer to many different adversaries. "Ha sheth'n" is also used to refer to the title of an angel in Yaweh's court, this angel is one of the "watchers" of mankind and his specific duty is to report the sin to god and act as a sort of prosecutor. This works with the definition of satan as "adversary," just in more of a legal-esque sense, with yaweh being the ultimate judge. This also puts the concept of christ being an "advocate" for humanity in better context.
I'm certainly not saying that Set is Satan (as some modern day theistic satanists believe). I just think the Christian concept of satan is an amalgamation of several characters from previous mythologies. While the word satan may have its roots with the god Set and some Christian stories may derive from Egyptian myths involving Set, the the New Testament character of Satan is much closer to that of the Egyptian Apep.
There is really no concept of Yaweh having an opponent before the Babylonian exile. Upon being rescued by the Persians and coming into contact with Zoroastrianism they may have adopted the Persians' concept of dualism. In Zarathustrianism the all benevolent Ahura Mazda battles the universally evil snake deity Ahriman continuosly. If Yaweh were involved in a similar sort of struggle, it could explain the Babylonian captivity. However, Yaweh is far from all benevolent, and Satan wasn't quite uniformly evil either. This makes Satan and Apep a much better match because both Ra and Yaweh inflicted brutal punishments on humanity and both Satan and Apep are evil chiefly because they oppose their counterparts.
There is quite a gap in time between the end of the exile and Satan really having a developed character. The vague dualism in Judaism eventually gave way to the developed character of Satan, but not until Christianity began to spread around the region. Satan likely picked up his Apep like traits when it spread through Egypt, about ten to twelve years after the death of Jesus. This was well before the New Testament was written, and notably the writer of the oldest canonical gospel, Mark, spread Christianity in Egypt. The notion of dualism was already there, and the very vague character of Yaweh's opponent left plenty of room for the locals to impress some of their pre-existing beliefs on the new religion.
Noah
