I was surprised to find that this emblem, commonly recognized as the symbol of the Islam, has actually acquired its relationship with the faith by association.
The star and crescent symbol itself is very old, its adoption into the battle-standard of the Ottoman Dynasty is mainly responsible for its association with Islam. The crescent moon was from ancient times the symbol of Byzantium (which became Constantimple and then Istanbul). After the city was taken by the Turks, A.D. 1453 it became the emblem of Turkish power and thus came to represent the forces of Mohammedanism.
Today, the star and crescent is widely accepted as a symbol of the Islamic faith, and is used in decorative arts, jewelry, and national flags- much like the cross in Christian countries.