Constantine had a lot to do with it.
The very early symbol for christians was actually the pentagram.
I realize this will shock a lot of christians who are oblivious to their faiths history.
So, here, see for yourselves:
http://en.wikipedia.org/wiki/Pentagram
""ChristianityThe pentagram is used as a Christian symbol for the five senses,[22] and if the letters S, A, L, V, and S are inscribed in the points, it can be taken as a symbol of health (from Latin salus).[23]
Medieval Christians believed that the "pentalpha" symbolizes the five wounds of Christ. The pentagram was believed to protect against witches and demons.[24]
The pentagram figured in a heavily symbolic Arthurian romance:[24] it appears on the shield of Sir Gawain in the 14th century poem Sir Gawain and the Green Knight. As the poet explains, the five points of the star each have five meanings: they represent the five senses, the five fingers, the five wounds of Christ,[25] the five joys that Mary had of Jesus (the Annunciation, the Nativity, the Resurrection, the Ascension, and the Assumption), and the five virtues of knighthood which Gawain hopes to embody: noble generosity, fellowship, purity, courtesy, and compassion.
Most Christians came to associate it with Satanism and subsequently rejected the symbol sometime in the twentieth century...."