Your brother makes an excellent point, which is something I’ve always found amusing about atheists: despite their professed godlessness, their values have an uncanny resemblance to those of their Judeo-Christian heritage. It’s extremely difficult to find an atheist who takes his/her atheism seriously and is willing to really think “outside the book” about what kinds of morality are possible and act upon those ideas. Very few can resist the moral inertia of their culture; those who do are generally thought of as “evil”.
As an example, I have no trouble imagining a scientific civilization that is brutally unegalitarian, aggressive, atheistic, polygamous, Eugenicist, Darwinian, etc. in its values; in fact this would be my preference. As another example, I've always found the Star Trek Mirror Universe more appealing and more human than the normal one. But to espouse such ideas in a society dominated by Judeo-Christian values is to be labeled a “Nazi,” etc. We in the West are mentally colonized by these values from day one, even (or perhaps especially) here on this forum of rationalist moralists, and it is very difficult to find truly free thinkers who are willing to challenge the prevailing ethos.
I can likewise imagine (and could prefer) a scientific civilization that is freely polyamorous, atheistic, Eugenicist, etc.
But "brutally unegalitarian and aggressive"? Why in the seven hells would I prefer to live in such a horrid place? Historical precedent indicates that the more unegalitarian the society the most horrid it is for the majority of its people. Aggressiveness is even more likely to lead to a horrible society. My limited personal experience confirms (my one-year military service being the the worst sub-society I've been in).
Perhaps...
I just had a long conversation with my brother, a devout Christian. With my help he has outlined the following argument why it might be good for me to follow Christian deontology:
What do you think?