It's me
I was raised Protestant salvationist, went Objectivist atheist, now realist-pantheist, but all the while retained cultural sympathy for salvationists (pro-just-laws, anti-infanticide, pro-objective-truth, pro-family, etc)
I think there are a few things happening
One is salvationists vs universalists
Salvationists believe the individual needs to attune himself to God (or Nature), universalists thinks everyone is saved and the individual has license to attune the world to himself
Universalists are nominally Christian but almost culturally unrecognizable, while salvationist Protestants, traditional Catholics, Orthodox and even Mormons seem to have a great degree of shared culture.
Another possibility is conversion prior to conversion. The individual joins the leftist cult (either Marxist in particular or the broader category of Hegelian). Eventually they see that Christianity is not compatible with their new faith and leave it.
Finally I think there is deconversion prior to conversion. The individual is convinced by "secular" forces (largely post-theist Universalists like the New Atheists) to abandon their religion, and having done so seeks some other worldview to make sense of things. The dominant options in our society are Christian traditionalism and the leftist cult. Having quit one, they try the other.
I was fortunate in that I joined the stridently pro-individual-reason Randian cult, and like many adherents (though sadly not all) deprogrammed myself of the Randian components that are most cultish (eg worship of Rand & uncritical acceptance of her ideas). I think this is one of the better cults to fall into. While those who remain in the most orthodox sect tend to be virulently anti-religion there seems to be quite a castoff of people who either remained religious while embracing elements of Randianism or who partially deconverted and are culturally sympathetic to traditional social structures. Of course, there are also Randians who by virtue of Rand's focus on self-pleasure become very libertine and essentially leftist.
