A secular Leftist-led (or infiltrated) organization can corrupt its members for a long time, by introducing and over-emphasizing secular Left priorities (egalitarianism, 'diversity', environmentalism etc).
When an organization becomes thoroughly corrupted by secular Leftism - as are the mainstream Christian denominations, the universities and schools, the legal profession, and every other powerful institution - then it may begin to subvert its own corruption.
The corrupting effect of liberal Christian groups is achieved because the nihilistic poison is sugared by elements of retained Christianity.
Indeed, the corruption would not be effective unless it contained elements of genuine good.
(The measure of the effectiveness of secular Leftist infiltration of the churches is therefore seen mainly in the older members of the congregations; people who used to be devout traditionalist Christians, but who have - by insensible, incremental stages - being led into unperceived apostasy.)
So all really bad things - Communism, Fascism, Modern Art - contain elements of good; and this is precisely why they are so dangerous.
(Thoroughly, explicitly and obviously bad things never get anywhere, never last long.)
Therefore, since bad must be mixed-with good; as institutional corruption proceeds towards complete destruction of all that is good, there comes a point when the attempt to corrupt becomes susceptible to a backlash.
Even the most wordly and secularized Leftist Christian church cannot help but include elements of Christianity. And at some point the congregation will themselves be so corrupted by the anti-Christian propaganda of these churches (in line iwth secular society), that these residual Christian elements (which cannot altogether be eradicated) will serve a positive Christianizing purpose.
(A member of the congregation might suddenly and unpredictably take notice of some of the words they speak or hear; they might even - more dangerously - pick-up and read an Authorized Version of the Bible or the Book of Common Prayer! They might, despite all misinformation and counter-incentives take it seriously!)
Thus, against a baseline of extreme corruption including propagandized and coercive inversion of truth, beauty and virtue; then even the slightest sniff of the Good (which seems impossible to avoid altogether) may well strike a positive response from the population that are being force fed on toxic swill.
I wonder if we may reach such a point?
The point when people stop actively wanting ever more evil under the pretence that evil is good; and (as is natural) begin again to regard good as good, evil as evil; truth as real; beauty as lovely?
Will Christian congregations then react with horror to their subversive Priests and Pastors; students be revolted by the careerist lies and cynicism of their teachers; media consumers become nauseated by their own manipulation?
It may happen.
Indeed, I don't see how it could be stopped - unless, and in so far as, human nature was itself changed.