Tip, when thinking of spiritualism, think of psychology and philosophy, don't think of religion. Metaphysics plays a large role in the two, and while it isn't scientifically grounded, it is still influential and plays on the idea people are limited by their senses to discover everything "scientifically", which has its merit in regards to things we cannot observe or study but can still very well exist.
Spiritualism Description In-Game:
-"It behooves us to remember how tiny we are, how pointless our lives are in this vast uncaring universe... what nonsense! The only truth we can ever know is that of our own existence. The universe - in all its apparent glory - is but a dream we all happen to share."
Fanatical Spiritualism Description In-Game:
-"Our science has proved that consciousness begets reality. We regard with patience the childlike efforts of those who delude themselves as they play with their blocks of 'hard matter'."
Spiritualism in-game, at least from how I interpret these descriptions, just involves the doubting of our (or aliens') senses, observation, and study, and the belief in a scientific reality that cannot be studied (things like souls, mind, consciousness, being, purpose, and relationship with all other life and matter can be explained both spiritually and materialistically, just with different conclusions).
Sorry, that might have gotten off topic, but basically with this explanation I just want to say that things involving Knights Templar/Jihadist units, or crusades, or organized religion, don't really fit "spiritualism" in-game, especially when spiritualism is more or less a competing school of approach to science rather than a complete opposition of science. There is a reason Paradox put things like psionic warriors rather than zealots and holy orders. Psionics is very much in the realm of metaphysical science (though whether or not it can possibly exist in real life is neither known nor important to the matter of how it is represented in game). Zealots would just be religious versions of nationalists, or even mercenaries if we are talking about holy orders (getting paid in salvation rather than money to fight other peoples' wars).