You know what I mean; maybe you have done it yourself. In discovering some aspect about some other denomination or practice of Christianity than your own - you mentally point the finger, saying something like 'But that's just Pelagianism'... or whatever.
The implication is intended to be that this is an old and dangerous error of theology; which arose, was exposed and was refuted in the early centuries of Christianity. A further assumption is that there can be 'nothing new under the sun' - that all modern attempts to say anything different from the past are not only mistaken in intent and dangerous in outcome - but merely some permutation of ancient errors.
My belief is that this is an ignorant way of proceeding. In the first place the ancient 'heresies' are (mostly) hardly understood - since we do not know what their adherents really believed. In some cases, the descriptions sound very like straw men: that is, accusations from enemies rather than genuine beliefs.
(The equivalent of a modern person trying to learn the truth about Christianity from the pages of The New York Times.)
But in the second place - and most importantly - this way of proceeding prevents any genuine comprehension and grasp.
I will give the specific example of Mormon theology. I am not, and never have been, a member of the Mormon church; however I have come to believe in the truth of Mormon theology (and indeed the validity of Joseph Smith as a prophet and the validity of the Book of Mormon as a scripture).
Even as a sympathetic investigator, it took me about five years before I grasped the distinctiveness of Mormon theology; because it is very, very different from mainstream orthodox Christian theology. It is, indeed, so radically different in its basic, metaphysical assumptions; that it entails a 'pluralist' philosophy outside the Western Canon - only being represented by William James, and such 'fringe' figures as Rudolf Steiner and Owen Barfield.
Now, I can easily understand that many people are perfectly satisfied by their mainstream orthodox Christian theology, and are not interested in examining the claims of some radically different and alternative conceptualisation of Christianity. I can understand that some people find Mormonism just too silly, trivial or boring to want to make any significant effort in understanding it.
Other people (most people) just aren't interested by really basic metaphysical philosophy, that challenges the most fundamental assumptions regarding the nature of God, reality, time etc. I think that such an attitude, here and now, is a very serious error; since most people have false and wicked basic assumptions of reality, derived from an evil secular culture; and for their own good and salvation they need to discover and examine these assumptions. But this need not have anything to do with Mormonism.
But I have, again and again, encountered the lazy and ignorant excuse that Mormon theology is 'nothing but' some kind of incoherent mish-mash of various ancient Christian heresies such as Arianism, Pelagianism and Gnosticism.
The accuser has pre-decided that this is the case because it 'must be' the case; and proceeds to fit Mormon theology into checklist categories of ancient heresies that he has derived from some secondary literature that purports both to know and to understand the faiths of these unorthodox early Christians.
This is no way to understand anything new. It is merely a recipe for ensuring that you will never learn anything outside of your existing compass.
In complete contrast; the way to learn some radically different way of thinking requires, in the first place, a lot of motivation - because the process is nearly always slow and requires sustained effort. But the effort needs to be directed at understanding from the inside out. It entails an act of sympathetic identification - to see things from the inside as an adherent sees them.
Lacking this, the understanding is a fake, 'machine-learning', flow-chart, algorithmic process - not worthy of being called understanding at all.
We need to see how it all fits together, how it derives from a few basic assumptions, how it makes sense in its own terms... Of after this has been achieved, can there be a genuine critique or development.
This is not a counsel of perfection. It is possible to do it, and I did it with Mormonism (after, as I say, about five years). Because this is what is actually being done by all
serious adherents - although they typically lack self awareness of it (that being the nature of humans). And you know when you've done it, pretty much; because serious adherents will accept your descriptions of them and their beliefs.
My point is that serious philosophical work is not a matter of pick-and-mix, of selection and combination. On the contrary it is a process of empathic identification and a development from-within, that is both intuitive and rational.
You cannot pick your subject, idle curiosity is insufficient motivation; rather, your subject will pick you - in the sense that your deepest concerns and wishes will seek understanding and answers; and this is what drives you to the necessary work and thinking.
But for me, this kind of philosophical grappling has been the greatest satisfaction and value in my intellectual and personal life; and why I regard myself as primarily 'A Philosopher' - despite that my work has been in the realms of science, medicine, literature and (most recently) theology - and almost never in the areas that professional 'philosophers' in colleges regard as academically significant.