The pentagram was used with astrological or religions meaning long before Christianity, and has been used by many cultures since. I'm not sure why Christians discarded it.
"I'm not sure why Christians discarded it."
The more I think about it, it probably is BECAUSE of the mutliple other religions making claim to the symbol that Christianity discarded it.
Opted for symbols fairly unique to Christianity: Sign of Jonah (fish, rising from death in 3 days and reference to miracle of fish); cup; cross, or all three together.
I mentioned in another post that the pentagram represents man -- head, arms, body. Another reason other cultures would find it a powerful symbol.