Picture of Magen David on the Old-New Synagogue Window of Prague is courtesy of "MindSpigot" who published it on Flickr.
The following paragraph is from Dr. Asher Eder’s book The Star of David, which was published in 1987 in English in Jerusalem by Rubin Mass Ltd. The publication here is courtesy of Oren Mass. This version includes corrections and new materials that do not appear on the printed version
What is more, also synagogues began now to show the hexagram, one in Hameln/Germany, and the famous Altneuschul in Prague, both built at the end of the 13th century.
Before going more into details about the latter, we should have a brief look into the historic context, that is, into the social atmosphere of then.
Christian anti-Semitism which got codified and legalized by Emperor Justinian in the 6th century, turned violent with the Crusades. The slaughter of Jews in France and Germany, and then in Jerusalem, are known. With the defeat of the Crusaders and the loss of Jerusalem, Christianity got a new trend altogether: “Earthly Jerusalem” was replaced by the idea of “Heavenly Jerusalem”. This idea found its architectural expression in the Gothic style of the cathedrals and churches, with the high extension of its pointed arches symbolizing the propensity heavenwards, and its raising towers with their tops in heaven. Within this trend, the Fourth Lateran Council of 1215 decreed that the Jews (and Saracenes) have henceforth to wear clothing which makes also a visible distinction between them and the Christians. The governments (as the “sword hand” of the Holy Sea) were to make regulations about specific dresses and the wearing of the “Jew badge” legal (this was done e.g. in the “Book of Laws” compiled by Alfonso X of Castile (1252-82); by Edward I of England in 1275; by Alfonso IV of Portugal in 1325). These badges, to be fixed on the hat or upon the chest, were mostly round, red or yellow in color, but there were also other signs of differentiation – e.g. the horned Jew hat; specified colors of the dresses; and indeed also badges in form of the hexagram. A royal decree, issued during the anti-Jewish riots in Spain at the end of the 14th century, forced Jews to wear a “red star with six points”, and a painting by Nuno Goncalves, now in the altar piece of St.Vincent/Portugal, shows such a six-pointed red star on the robe of the chief rabbi of Portugal, just over the heart.
Within the frame of these historic developments, we should have now a closer look into the key role the above mentioned Altneuschul of Prague was to play in the slow transition of the hexagram from a random sign to a symbol of the Jewish people.
Built at the 13th century in the early Gothic style of then, it shows rosette-like hexagrams in its two round front windows.