There are endless good ways to live. Part of the beauty of our species is that new ways to live well are perpetually being realized by us, and that we have created civilizations in which, occasionally, circumstances allow at least a few people to follow their hearts, to appreciate, to love, and to create, without being hindered by arbitrary rules or fears of being outcast, marginalized, or extinguished. Such eruptions of freedom and goodness may be rare and short-lived, but they show us the best of what we are, and what kind of world we might live in if we have the vision, the cunning, and the strength.
I am not likely to change my mind about this: I don't think there is any objective morality or any ultimate justification. Some people seem to think that this leads to "cultural relativism", the idea that we cannot and should not judge cultures outside of our own, or people attached to those cultures. This is a fallacy: if there is no objective morality, there is no objective imperative to respect other cultures. Still, people who don't believe in any objective morality are happy to hold their own commitments and respect those of others, and are entirely capable of choosing to stand up for their commitments when conflict is imminent. Moral skepticism/nihilism/relativism need not lead to paralyzing indecision any more than it must lead to cartoonish indiscriminate violence.
When do I respect cultures that differ from my own, then, in belief or in practice? To what extent do I tolerate, respect, or even celebrate difference, and when do I despise and condemn it?
I hope the first paragraph offers a clue: to the extent that your culture allows its members to live well in their own way, I consider it respectable. To the extent that it does not, I consider it shit. This is not a matter of whether your society is predominately Christian, Muslim, Jewish, Hindu, atheist, marxist, capitalist, or whatever; it is a matter of whether people living within it find the freedom and the resources to live the lives they think are best. If your culture can't support that, your culture sucks.
That said, it's not always easy to draw the line. It shouldn't be too hard for any of us to remember a time when we've misjudged another individual after failing to see why they're living the way they are, and the difficulty in accurately assessing another person's point of view is multiplied in assessing the value of a whole culture for the people within it.