Are the mainstream Christian churches substantially corrupted by secular Leftism? Yes.
Is this a fact or mere opinion? It is a fact.
How would we know? See below:
One test is very simple: and the test is exactly what the secular Left say it is. Where does the Christian church stand on the Hot Button issues of the day.
For more than 50 years the secular Left has rolled-out a sequence of Hot Button moral issues to create conflict with the Christian Churches - and these are almost all to do with sexual morality. Each of these sexual issues then becomes a political cleavage-point or litmus test for good versus evil as defined by the Left.
And before the sexual revolution of the mid 1960s, there were analogous challenges spread out over previous decades and indeed over a couple of centuries: the proposal to use secular historical scholarship to over-rule scripture was one of the first and deadliest; natural selection versus creation was a later one where a scientific metaphysical assumption was put forward to over-rule the Christian understanding of the human condition; and at various times abolition of slavery, pacifism and socialism have been the major issues.
In retrospect we can discern that each of these issues was a wedge - and that all were significant.
Acceptance of even one of these issues acts like a wedge, because the new principle expands incrementally to break-apart the unity and coherence of Christianity in the way that wedges were used to break-up a tree trunk.
This applies to the issues of sexual morality which dominate church-secular controversies today.
All of the Hot Button issues have been chosen by the secular Leftist system (whether chosen by blind, impersonal, systematic summated social forces, or chosen by evil intent) for their long-term potential to destroy Christianity (or subvert and rewrite it - which is the same thing).
None of these issues are insignificant, as the secular Left realizes - which is why the secular Left pursues these matters with such strategic zeal and ruthless aggression.
The secular Left are hard-line fanatics, the Christians try to be 'reasonable' and meet them half-way - with predictable results...
In no instance will small-scale 'compromise' changes be acceptable to the secular Left - since their agenda is revolutionary and destructive - all small changes will simply open-the-door to further changes in the same direction.
Now, we now that this is the case; looking back we can see the same pattern again and again.
There is nothing for Christians to debate here; because debate implies the possibility of change and to be 'reasonable', to seek a mutually acceptable compromise, is simply for the church to surrender and to weaken itself for the next attack.
(The argument goes: "if you have already agreed to that - then logically you must also agree to this which is merely a consequence of that.)
All this the secular Left perceives with absolute clarity.
Yet Christians do not.
Christians continue to pursue the policy of appeasement, conceding and conceding to an aggressor who has stated quite clearly that he wants nothing less than your annihilation - consequently the mass majority of ever-yielding Christians are in a perpetual state of disappointment and are continually aggrieved that their decency is not being appreciated by their enemies - constantly shocked that their enemies are being so unfriendly!
But Christians need to wake-up.
Christians absolutely must reject the secular, Leftist anti-Christian agenda in its totality; including to go back and undo or reverse all the concessions and yieldings to secularism made over the past centuries.
All of them.
Because all the Hot Button issues were chosen by secular Leftism, all are wedges, all are capable of great destruction, all need to be decisively rejected, eventually all need to be removed.
Individual Christians need not, should not, try to understand the modernizing agenda from an assumption that it is 'well-meaning': it is not well-meaning - even when the individuals who pursue it are nice and decent people, it needs to be kept in mind that they are nice and decent people in the service of evil.
All and any future proposed 'reforms' originating from the secular Left therefore must be refused categorically and finally, without argument.
And any and all of the changes to Christian theology, morals, scripture, liturgy (etc.) originating from this sources need to be reversed.
We moderns have, we should recognize, been corrupted, such that we are incapable of 'judging for ourselves' - how can hedonistic chaotic addicts with the attention span of a gnat, near-zero Christian knowledge and even less sanctity, judge anything 'for ourselves'?
It is often said that scripture, reason and tradition ought to guide Christians: quite true. But we modern cannot read scripture, and our scriptures have been re-written; we cannot reason because we are ignorant, inattentive and incompetent; therefore we must focus primarily on tradition where this is clear, and tradition is crystal clear on all the Hot Button issues
- and we must let tradition guide our understanding of scripture and reason (i.e traditional understandings of scripture, traditional modes of reasoning).
Therefore, we moderns must be guided primarily by tradition, by ancient tradition, by tradition originating from before modernity - and especially when this tradition is clear and unambiguous among the Holiest of our Christian ancestors.
NOTE: This perspective on the nature of modernity can be seen throughout the work of Fr Seraphim Rose.