When has this been stated? Yes, there's such a thing as a disembodied consciousness maintained in the form of an energy matrix of one sort or another, but even theologians wouldn't agree on whether a sentient consciousness was the same thing as a soul. We've heard characters expressing their belief in a soul, but that doesn't constitute proof that it's treated as something that literally exists. ST has always been far more agnostic on theological questions.
This would just be semantics on my part, but my view is it's less semantics than dealing with theological issues in a secular and humanist manner. Apollo is a god from ancient Greek myth, alien or not, but the Enterprise crew doesn't need him because they've outgrown Greek-style divinities. The katra isn't a soul in the Christian manner but one could certainly argue it's a soul in the "Ka" and "Ba" manner of Ancient Egypt.
One could certainly be an atheist in Star Trek (and many characters are confirmably so--it's also how Roddenberry envisioned future humans) but I think something approximating a soul exists in Vulcans (not necessarily humans).
Still, you're right, my statement was premature and misleading. "Spock's psychic presence" was transferred to "Young Spock" and that "rebooted his harddrive" so to speak if that sounds better.