When I was an atheist (i.e. for most of my life) - I had several views about happiness, and its status as a goal for living. Yet, happiness, in one form or another, was a kind-of ultimate index for my life and for human society.
Sometimes it was a long-term sense of deep personal fulfillment that I sought - sometimes very much the here and now, because it was much more certain than the future and contingent.
Sometimes I was seeking positive happiness, but often I was mainly seeking to avoid suffering; and this negative-happiness has always been a strong, and growing, element in secular left morality.
Indeed, modern, mainstream and hegemonic leftism has given-up on the old utopian ideals, and is focused entirely on the (supposed) objective of reducing various forms of suffering in an ever-expanding array of 'victim groups'.
Thus 'happiness' is sought by purportedly eliminating the supposed causes of misery - such as sexism, racism and *phobias.
But there has always been a view among atheists that happiness ought Not to be the main aim of life.
This has been argued both on the pragmatic basis that aiming directly at happiness doesn't work as a strategy for becoming happy; because happiness is a (temporary) by-product of other kinds of aim. And also because other kinds of aim should come before happiness - a modern example would be 'social justice'; and the assertion that happiness should be sacrificed to its attainment.
Yet, further analysis will find that the purportedly not-happiness aims, will always boil down to a happiness justification - thus, the primacy of 'social justice' is argued on the basis that it will make the sufferers from injustice less miserable (or more happy).
And, indeed, if my own happiness is regarded as selfish, while some more general happiness is regarded as more altruistic - there is the problem of justifying altruism as an ethic when it impairs my here-and-now happiness For Sure, on the basis of only conjectural and probabilistic improvements of happiness in other people.
It seems much more certain and solid to pursue personal happiness, than to make guesses about the possible future states of others in response to conjectural and multi-step causal effects of my present actions.
In other words, if any happiness-based ethic is termed utilitarianism; then we can see that all secular moralities are - sooner or later - made into versions of utilitarianism. And utilitarianism ultimately depends on whether it makes Me happier, here and now - because if it does not, then we might create more misery i the short term, in the attempt to reduce it in the long term.
But how about religions, and how about Christianity in particular? As an atheist I used to regard Christianity as merely another utilitarian ethic; which aimed at the positive happiness of Heaven and avoiding the negative misery of hell... But with the Christian enhancement of happiness displaced to a supposed eternity that purportedly came after death; rather than to the immediate here-and-now of this mortal life.
If Christianity is factually true, and if one could be reasonably confident of attaining Heaven and/or avoiding hell by being a Christian - then it would be rational to trade temporarily sub-optimal happiness for permanently greater happiness and/or avoiding the torments of hell.
However, even if Christianity was true; I did not regard it as a 'higher' form of morality; because - by the above analysis - it was still reducible to a selfish desire for happiness.
Are we then doomed inevitably to be utilitarians of one sort or another - is everything truly reducible to an 'hedonic index'- and is happiness therefore just a matter of feelings?
Well, it is only true if these are our foundational (metaphysical) assumptions. If we define happiness as a feeling, and if we decide that optimizing pleasant feelings and minimizing aversive feelings ought-to-be the prime goal of life: then we have already decided that utilitarianism is necessarily true, and there is no 'higher' goal in life than the hedonic.
But if we assume something else than happy feelings is primary, then happiness will not be primary.
For instance, if we regard happiness as an objective state of being, rather than feelings, then we will get a different kind of ethics altogether.
Or, if we regard happiness as secondary - such as being a psychological reward for virtue, or for creativity - we get the idea of happiness as a (fallible, but potentially valid) form of guidance; rather than an end in itself.
Or we might assume that happiness is not reducible to one variable, to a single index; but that there is instead a collection, spectrum or hierarchy of positively rewarding states of being - with different degrees of rewardingness.
We might, as Christians, posit that there are spiritual forms of 'happiness' that are not descriptive of body states; but independent from them; and these spiritual forms of happiness might have other properties - such as being indivisible from Christian values (such as truth, beauty and virtue).
This thought-experiment reveals that the mainstream modern and materialistic understanding of happiness assumes that it is dependent on the body. This 'transhumanist' concept of happiness (very common nowadays, albeit mostly implicitly) also assumes that happiness is something that can be (and should be) detached, separated-from other values - and pursued directly.
For instance - if happiness is regarded as a feeling, and separable; then it can be enhanced by modifying the human body (e.g. with environmental engineering - such as 'social justice', or euphoriant drugs, or potenitally by genetic engineering) to generate whatever happy/ not-suffering body states are preferred.
But the validity of this project depends on both the assumptions with which 'happiness' is set up as a goal, and the assumed definitions of happiness.
In other words; we can conclude that the atheist idea of Christianity as just-another version of utilitarianism is not rooted in any kind of 'objective fact' or 'observation'- but instead depends on atheist assumptions.
If the atheist assumptions are false, and are rejected, then the 'happiness' of resurrected eternal life in Heaven may be a different thing altogether than the aim-at situations of materialistic utilitarian leftism.