(From my personal experience...) Atheists think it is absurd and irrational when religious people talk about eternal life beyond death as necessary for a meaning in life; because if this mortal life lacks meaning, then why would a mathematical extension of it provide meaning?
(For atheists...) It is like multiplying zero - no matter how many times it is multiplied then it is still nothing. If mortal life has zero meaning then in can't be increased by duration; but if mortal life does have a meaning then there is no absolute necessity for eternal life. ('One life is enough!...')
From an atheist metaphysical perspective, that is - with atheist assumptions that a human life is, objectively, simply an incident in biological history, this argument appears irrefutable.
But if we take a religious perspective which assumes that reality was created, for a reason, by a deity who is our father - then we begin with a picture of a vast and unfolding story, stretching across 'eternity', in which we are participants. For Christians we are - by a family metaphor - both individuals and members of this story: personally concerned for our-selves and also for others.
From this perspective, mortal life has two aspects - the first is that one human lifespan - whether 7 minutes or seventy years - it is of microscopic duration compared with the timescales involved in the great story of creation (indeed, if eternity stretches without end - a lifespan is, relatively, of near-zero duration).
So mortal life requires consideration in the perspective of eternity in order properly to understand it. In other words, the atheist has misframed the question - because he is viewing eternity from the perspective of mortality - assuming the validity of the finite span of mortality and challenging the validity of an eternal perspective; when the correct procedure is the opposite way around.
But on the other side, Christians ought to remember that mortality was also experienced by Jesus Christ - and this indicates that the experience of mortality (however brief in comparison with eternity) is very important, presumably necessary, for the fulfilment of the divine story.
Non-Christian religions are often good at explaining the eternal perspective, and arguing in favour of an eternal perspective which shrinks (sometimes to microscopic levels) the importance of mortal life. But they tend to have trouble explaining why mortal life is of any value at all: why bother with it?
Mainstream Orthodox Christians also often have the same trouble - but this is not intrinsic to Christianity, but is a consequence of building-in inappropriate Greco-Roman derived philosophy, and then seeing Christianity through its lens.
The clarity of the Gospels should remind us that mortal life needs to be understood in a divine and eternal context - that life-on-earth is a relatively brief and temporary phase; but also that even the briefest of mortal lives is also vitally important both to our personal selves and also to the great plan of things.
