You will find more information if you look for "pentagram" than "pentacle". If I remember correctly, John Michael Greer has a decent entry on the pentagram in his The New Encyclopedia of the Occult.
The pentagram (not necessarily encircled) entered the Western esoteric tradition through the Ancient Greek Pythagoreans, who associated it with health. Since then, it has primarily been associated with magic--primarily power and protection. During the Christian centuries, it was generally perceived as a Christian symbol (as you just discovered) because that was their context. It was *never* associated with pagan religion until the latter part of the 20th century, and then only because British Traditional Wicca--and then other forms of Modern Pagan Witchcraft--took it from Ceremonial Magic.
Pentacle, on the other hand, derived from a French phrase for "hung from the neck" and was used for *any* pendant containing a sigil or similar design in ritual magic evocations. The Golden Dawn extended the use to refer to one of their elemental tools: a disc displaying a hexagram against a background divided in quarters, each quarter in the color of one of the four elements in the sphere of Malkuth. (They used the alternate term "lamen" to describe ritual pendants.)
The use of "pentacle" for a pendant or altar object prominently displaying a pentagram (almost always encircled) appears to have arisen within Modern Pagan Witchcraft in the latter part of the 20th century. As far as I can tell, it was a conflation of 1) the ritual use of pentagrams inscribed in the air at the quarters in British Traditional Wicca; 2) the mention of "pentacles" for objects inscribed with sigils in the Gardnerian First Degree Initiation Ritual; and 3) the use of "pentacles" as a title in the Rider-Waite Tarot for the suit of discs or coins, showing discs displaying an encircled pentagram. I am unclear if this conflation began in BTW and migrate outward, or began in Wiccan-derived Craft and was later used in BTW.
It is my strongly held opinion that, as the pentagram was historically associated with magic rather than pagan religion, the pentacle symbol should only be used as a religious symbol within Modern Pagan Witchcraft. Other pagans are strongly encouraged to use a symbol relevant to *your* paganism. (E.g.: a triskell or triquetra for Celtic pagans or a Thor's hammer for heathens.)