Most people who have thought about it (actually, that isn't many people) would agree that Leftism (or 'liberalism') is self-destroying.
Therefore Leftism is an ideology which is transitional between what went before and what comes after Leftism.
In other words, Leftism is destructive of what went before, and supportive of what comes after.
Well, Christianity is what came before - so, is Leftism fundamentally destructive of Christianity, or only accidentally (and non-essentially) so?
While there was some space for disagreement about which until about 40 years ago, it is now as clear as anything ever is that Leftism is anti-Christian in its root and motivation.
The strength of Leftism's anti-Christianity is that it is consistently and coherently anti-Christian without even being consciously aware of the fact! Even (or especially) when denying the fact.
Let us learn from this. Let us not look to what Leftism says about itself - because Leftism's self-knowledge is, ahem, very limited - but to what Leftism does coherently and strategically.
The penny drops.
Leftism imagines it is anti-religion and in favour of stuff like sexual equality, sexual freedom, peace, prosperity, comfort and so on - but does nothing at all to safeguard these supposed values, indeed destroys them; using them merely as clubs with which to beat Christianity. But only Christianity.
But Christianity is not the only world religion, ideology, or political system; and is not one which has grown manyfold its share of the world's population in a century plus under Leftism; nor has Christianity made great gains in power and territory and influence around the world, sponsored by Leftist nations. Nor has the Christian way of life been supported within all Leftist nations. Nor have large numbers of devout Christians migrated to Leftist nations. And Christianity is experiencing near total suppression, destruction and displacement from its historic heartlands.
Never mind what Leftists say or believe about themselves and their motivations; they are liars who do not even believe in the reality of reality; and insofar as they acknowledge the good it is only to invert truth, beauty and virtue.
Don't listen to them - look at what they do consistently, coherently, both tactically and strategically - and you may understand Leftism as being in its essence transitional, a temporary stage between the Christianity which it destroys and en route to something else which it encourages, and will take-over whatever remains after Leftists have done their work.
Of course, it would be ridiculous to imagine that any such plan was human in origin.