People are always accusing each other of being dumb, crazy or mastered by wishful thinking - in fact, I personally do it a lot on this blog.
But aside from he said/ she said - what actual objective evidence could there be that somebody's ideas were false, a delusion?
Mostly it is that false ideas do not adequately model or predict reality - so that someone whose ideas are false finds that things based on those false ideas don't work.
So the objective sign of false (potentially) ideas is maladaptive behaviour: but what behaviour counts as maladaptive?
We have to be careful here, because to be adaptive to 'the world' is merely to be 'worldly' - to seek short term happiness, status, pleasure and the rest of it.
When we are talking about very general ideas, such as religions and ideologies - then the category of 'does not work' needs to be independent of those ideas or else the argument becomes circular.
In particular, the truth of ideas cannot be judged by the here and now emotional state of the people who hold them; and neither can it be decided by their popularity, prestige, 'success' or whatever - because that would be to beg the question.
To discover whether ideas are maladaptive (hence probably wrong) I think we need to focus on basic, biological outcomes such as:
This is the most significant. Any society (nation, religion, religious denomination, or atheism) with average chosen fertility below the minimum replacement level - a birth rate less than two per woman, especially in a situation where it is probable that more than two offspring could successfully be raised to the age of sexual maturity) is deluded.
This is the only strictly biologically-valid sign of maladaptive ideas, since a fertility rate less than two is always a sign of severely reduced (negative) 'fitness' - nothing could compensate for sub-replacement group fertility.
This, of course, labels the whole of the West as fundamentally deluded; including almost all of Christianity.
But, within the West, the only groups who choose above replacement fertility are religious. One (or more than one) of these groups might be un-deluded - but all sub-fertile groups are necessarily deluded.
I also suggest the following as probabilistic evidence of delusion:
1. Significant/ increasing rates of deliberate and purposive self-harm, self-mutilation (including tattoos and piercings), and repeated suicidal 'attempts' (parasuicide) are un-biological, maladaptive and evidence of delusion - and these are especially significant of delusion when these are among women. Those who behave in this aggressive way towards their own bodies are, to that extent, objectively deluded (unless, of course, they explicitly repent, relabel, and repudiate their past behaviour).
2. Focusing exclusively on secondary issues to the exclusion of primary issues.
This is very characteristic of modern society; of Leftism and the Mass Media - and so common in everyday life as to be almost invisible.
(Atheists are very predominately leftist/ liberal/ socialist/ communist/ and in favour of the sexual revolution.)
It includes things like ignoring the issues of food production when talking about agriculture, and ignoring education when talking about schools and universities, and ignoring truth when talking about research and scholarship, and ignoring the production of essential goods and services when talking about economics, and ignoring the prevention and suppression of crime when discussion the police, or ignoring defence when talking of the military.
Instead all attention is focused on secondary issues such as equality, Social Justice, diversity, Fairness, aesthetics, Rights... well, pretty much anything except the primary issue.
How can this be seen? Well it is a matter of responses.
For example, if anybody brings up in public discourse the question of whether a scientist is truthful, or whether a scientific field is honest, or whether modern scientific organization systematically encourages and rewards lying - then this line of enquiry is met by silence, horror, aggression, denial... but never is this issue allowed to become acknowledged as vital to the basic purpose of science.
Because of this exclusion of what is a primary (indispensable) requirement for science, modern science is deluded: objectively deluded - and this is not merely a matter of opinion.
The same with respect to the military and police and medial schools when their recruitment and selection processes focus on factors like sex, class and ethnicity - they are objectively deluded in their attitudes.
There are probably some others.
But the general approach shows that we live in a deluded society (i.e. our secular society is deluded) - and indeed almost all non-religious people, and most religious people are objectively deluded.
Self-identified atheists in The West are (I would have thought very obviously) among the most deluded people of all (and among the most deluded of any time in human history) since, as a class, they most strongly exhibit the above characteristics.
The fact that Western atheists regard religious people as deluded because they believe things that atheists happen to regard as absurd or silly or dumb or evil is irrelevant - this is just a matter of opinion.
And these are, after all, the opinions of objectively-deluded people - so they must be treated with special caution and scepticism.
Unless atheists could show that religious beliefs are objectively associated with maladaptive outcomes - such as sub-replacement fertility, self-mutilation or inability to perceive primary reality - then these accusations mean nothing - they are just exhaled warm air...