...is that Christianity is a ridiculous fairy tale when compared with the real problems of life such as war, starvation and disease.
And that to preach the gospel to people suffering from w, s & d is therefore insulting, disgusting and obscene when what these people need is peace, plenty and health.
And yet Christianity arose, grew and has reached astonishing levels of devotion in situations of human fear and suffering far beyond anything suffered by modern atheists, or indeed by anyone alive in the modern world (except for certain diseases).
Many of the greatest works of Christianity (by the Disciples, the Apostle Paul, Boethius...) were actually written under conditions of imminent torture and death; many others under conditions of extreme voluntary ascetic deprivation - fasting, freezing cold or parching heat, isolation...
What really is insulting, disgusting and obscene is to preach atheism, and therefore nihilism, to people living under conditions of war, starvation and disease - because this is to preach that their lives have been, are and will be meaningless and purposeless; and that they and everyone who they know and who ever have lived exist alone in a universe of pain from which any relief is partial and evanescent; and then they and everything they value will be annihilated leaving no trace.
Atheism is merely an indirect way of preaching the necessity of immediate despair and urgent suicide.
Nihilism may clear the path to a wider range of gratifications and diversions for healthy but jaded hedonists living trapped inside abstract fantasies and under conditions of peace, prosperity and comfort...
But atheism is not just stupid but indeed is pure poison - existential torment - for people living in contact with the real world - with war, starvation and disease.
Of course there is another common, and opposite, atheist criticism of Christianity - which is that people are Christian only because of war, starvation, disease and other extreme hardships - and this is why they invent and cling-to such nonsense. But that when you get intelligent and knowledgeable people living in peace, prosperity and comfort and with leisure to reason and critique; only then do they become able to reject self-gratifying fantasies, and can at last see through the illusions of religion, and perceive the meaninglessness and purposelessness of reality.
In one view Christians are seen as avoiding harsh reality by fantasy; in the other view Christians are seen as overwhelmed by harsh reality and therefore unable to be rational.
In one view atheists are self-perceived as braver than Christians in terms of honestly facing-up to the horror of reality; in the other view atheists are self-perceived as more rational than Christians precisely because their reality is much pleasanter and they are not required to be brave.
Atheists will often oscillate between these two opposite criticisms.