Information from http://www.us-israel.org/jsource/Judaism/star.html
states:
"Star of David
The Magen David (shield of David, or as it is more commonly known, the Star
of David) is the symbol most commonly associated with Judaism today, but it
is actually a relatively new Jewish symbol. It is supposed to
represent the shape of King David's shield (or perhaps the emblem on it), but
there is really no support for that claim in any early rabbinic literature.
In fact, the symbol is so rare in early Jewish literature and artwork that
art dealers suspect forgery if they find the symbol in early works.
Scholars such as Franz Rosenzweig have attributed deep theological
significance to the symbol. For example, some note that the top triangle
strives upward, toward G-d, while the lower triangle strives downward, toward
the real world. Some note that the intertwining makes the triangles
inseparable, like the Jewish people. Some say that the three sides represent
the three types of Jews: Kohanim, Levites and Israel. While these theories
are theologically interesting, they have little basis in historical fact.
The symbol of intertwined equilateral triangles is a common one in the Middle
East and North Africa, and is thought to bring good luck. It appears
occasionally in early Jewish artwork, but never as an exclusively Jewish
symbol. The nearest thing to an "official" Jewish symbol at the time was the
menorah.
In the middle ages, Jews often were required to wear badges to identify
themselves as Jews, much as they were in Nazi Germany, but these Jewish
badges were not always the familiar Magen David. For example, a fifteenth
century painting by Nuno Goncalves features a rabbi wearing a six-pointed
badge that looks more or less like an asterisk.
In the 17th century, it became a popular practice to put Magen Davids on the
outside of synagogues, to identify them as Jewish houses of worship in much
the same way that a cross identified a Christian house of worship; however, I
have never seen any explanation of why this symbol was chosen, rather than
some other symbol.
The Magen David gained popularity as a symbol of Judaism when it was adopted
as the emblem of the Zionist movement in 1897, but the symbol continued to be
controversial for many years afterward. When the modern state of Israel was
founded, there was much debate over whether this symbol should be used on the
flag.
Today, the Magen David is a universally recognized symbol of Jewry. It
appears on the flag of the state of Israel, and the Israeli equivalent of the
Red Cross is known as the Red Magen David."
