>Moreover, I think that the fact that grownups can talk about supernatural religious claims as if they are reasonable and true things about the world is one of the root causes of what happened in Paris.
I don't disagree with this. However I think another root cause is that you can influence people to do terrible things when you convince them it's in the name of peace or freedom etc. Humans have no problem getting the ideas of nationalism into the heads of Neo-Nazis; no supernatural claims are needed here. Religion can be used as a tool, and a tool can be used in multiple ways.
>But you've admitted (and I know full well) that there is a great deal of supernatural nonsense tied up with it, for most real-world practitioners.
Whether you think it's nonsense is dependent on whether you believe the Buddha was enlightened or not. I have faith that he was, and that the things he related to people are a path to become enlightened ourselves. When I accept this, it leads to the acceptance of the "nonsense" - the things that you must take purely on faith until doubt has been eliminated - and doubt is eliminated through mindfulness (concentration and insight) meditation. I have faith until I get there. If I don't get there and I still have faith, I'm not unhappy about it.
I can spend my time practicing the Noble Eightfold Path, the practice of which results in happy outcomes for myself (real or not) and being nice to other people, causing no suffering or death or ill will nor bad feelings et cetera. If I can accomplish that, then I'll be happy.
I don't even need Buddhism for this; I could go out and follow the principles, but I will say it will feel incomplete. There are various things that may stand in the way. For example, if I view there being no consequences to clinging and attachment (if I didn't believe in karma), what reason would I have to eliminate clinging? With clinging, I'm still unhappy etc. and my own unhappiness means I will have very little happiness left to give for others. The belief is a net benefit to me, and I think to the people around me.
>Shrouding it in important-sounding, mystical East Asian language and symbols is just silly. Like people who have "katanas" hanging over their mantles.
I don't know of any alternative terms to nibbana and kamma aside from "nirvana" and "karma". I write them like this only because this is how they are written in the romanisation of the Pali language, the language in which the Buddha's discourses are written in. To say "karma" leaves the interpretation slightly more open to the meaning in Hinduism, which I believe is a little different.
Kamma can, as far as I know, best be described as "cause and effect" on a very large scale. The idea that actions have consequences; the idea of "bad karma" and "good karma" is tied up really in interpretation. You can see the consequences as good or bad. From the Buddhist perspective, it's a "law" of the universe, not dissimilar to the laws inside physics that model the universe. I suppose you can use "cause and effect" as a secular term for this, but if you dismiss the concept of other worlds then using it as "cause and effect" would be fine.
Nibbana is more difficult to describe in a "secular" way, as there's no way of knowing that it actually exists unless someone tells you. You need to have faith in it, or actually experience it in order to see the reality of it. In fact, it's said that nibbana is beyond words, so any words one uses to describe it are approximations; e.g, from Samyutta Nikaya 43:
The unfabricated, the uninclined, the truth, the far shore, the subtle, the very difficult to see, the unaging, the stable, the unintegrating, the unmanifest, the unproliferated (nippapancan), the peaceful, the deathless, the sublime, the auspicious, the secure, the destruction of craving, the wonderful, the amazing, the unailing, the unailing state, Nibbana, the unafflicted, dispassion, purity, freedom, the unadhesive, the island, the shelter, the asylum, the refuge, the destination