The pentagram is a religious symbol used primarily by Wiccans today, it was historically a Sumerian astrological symbol denoting the planets. Pythagorean believed it to represent the 5 elements of life and it was used as a symbol of life throughout Greece.
Eliphas Levi continued to use the pentagram in much of his art, he was also the first to show it upside down in his Sabbatic Goat drawing which later became known as the Baphomet. He considered that if it denoted life, then it denoted good, and the reverse would thus be the reverse. He is famously quoted as saying:-
"Let us keep the figure of the Five-pointed Star always upright, with the topmost triangle pointing to heaven, for it is the seat of wisdom, and if the figure is reversed, perversion and evil will be the result."
The Christian Church used the pentagram as a symbol of health and vitality up until the the 1970's where it was rejected as it was too readily associated with Satanism.
The Pentagram in Satanism is encircled twice, this is used in the same manor as Pythagorean symbology to represent a void, this symbol by Christianity is to represent a lifeless place where the fallen angels where sent in Hell. The three downward facing points are to denote a rejection of the Holy Trinity. The entire symbol is designed to be opposing, and a mockery to Christianity.