And in spite of the best efforts of those apologists who tirelessly defend the faith, pagan beliefs are daily being promoted (wittingly or unwittingly) by professing Christians.
For instance, one of the pagan beliefs about the soul is back in popular fashion again today in some religious circles.
Pre-existentianism is the term used for the idea that the souls of people exist in heaven long before their bodies are conceived in the wombs of their mothers, and that God then brings the soul to earth to be joined with the baby's body as he or she grows in the womb.
But this view is not held by either Roman Catholic or Protestant theologians and is dangerously akin to ideas of reincarnation found in Eastern religions.
There is no support for this view in Scripture. Before we were conceived in the wombs of our mothers we simply did not exist, in spite of what pagans would like to believe.
There is a great passage, in St. Athanasius somewhere, I think, that my wife really likes. He basically says that no matter how tired we get of doing it, and how ridiculous the things are that we are having to refute, we have to go through the drudgery of doing so, because there will always be someone foolish enough to believe them if they are allowed to pass unrefuted.
The trick, of course, nowadays is to accomplish this without seeming too harsh or shrill -- in our society that makes for an instant "ear-closing event..."
Of course, the pre-existence of souls is nothing new as a Christian heresy. No less an intellect than Origen -- who was incredibly brilliant, very pious, and whose process of "doing theology" underlies much of patristic thought -- fell prey to entertaining the possibility of this idea. He was condemned (posthumously) by the Church for it.
Pre-existence of souls is taught by the Mormons, incidentally, but has, as you say, absolutely no place in Christian teaching and has been condemned by the Church specifically. As to re-incarnation -- that is yet another step further away...