The difference between ethical atheism and nihlism is that ethical atheists have more faith

[jstrodel]

Well given that you believe that there's a higher power, I rather think that you will think that human happiness is base in comparison to that higher power.

Our consideration that human happiness is the point of morality is because morality deals with the well being of human beings, that they are safe, happy, and healthy.
Christianity is one of those religions that tends to have a leaning towards self reproach, and the reproach of other people, so it's understandable that human happiness is not the highest priority.

We're not presupposing anything, though, we're basing our actions and definitions upon what is demonstrable. We can't be expected to take into consideration things that do not have a proven effect on the situation.
That's be like saying "Oh I better not go swimming in this lake, it might upset the merpeople."

Russell Kirk asks a good question: "What is happiness". Aristotle's conception of happiness was very different from utilitarianism, Mill's understanding. Aristotle's sense of happiness, eudaimonia is related to a teleological view of human nature. Happiness is directed towards satisfying the highest possible virtue. This is different than arguing for "happiness" based on some artificial construction of a social good.

The kind of happiness that receives its definition not from teleology (unacceptable to the atheist) but from some sort of social construction (as in J S Mill's the greatest good for the greatest number or some sort of liberal political morality) is at its roots authoritarian. It is a way to force others to accept the idea of happiness that the society defines. What is happiness? If it is defined in a non absolute way, happiness is just one mans will to force another to accept his will.

What produces happiness and creates happiness is totally different in different societies and environments. Happiness is also distinct from pleasure. Few wise people would say that a society that mostly pursues pleasure will in the long run achieve happiness, that holds true on an individual level as well.

Christianity is not opposed to happiness, but it is for joy. Joy is similar to the Aristotelian concept of happiness (though it has been years since I have studied that), that flows out of correctly operating human behavior. This is totally different from from defining happiness in a way that makes it a made arbitrary prioritizing of certain types of emotions or feelings over another - e.g. happiness is the right of all people to own a TV.