TPAM should know that that is mostly
not true; that is an idea held mostly by revisionists and radicals. Most symbols were made themselves, and only a handful were repurposed from pre-existing beliefs. The Ichthys, the Triquetra and a couple of barely-used symbols (the phoenix, the tree of life, etc) come from either pagan or Jewish traditions, but the cross, crucifix, icons, the lamb, the alpha and omega, the "IHS", and the "keys" are originally Christian.
As for the Chi-Rho, it was used to mark important passages in pagan texts, but it was an essentially secular symbol until Christians repurposed it. As for the Chalice... it is Christian, but Wiccas and Neo-Pagans are trying to re-purpose IT, mostly without success.
There is a whole site full of Christian symbolism, and most of the symbols there are either original to Christianity, or are from Judaism. It even says which ones are pagan, and there are not that many. Take a look:
http://home.att.net/~wegast/symbols/symbolsc/symbolsc.html