I understand your sentiment, but 1) you're making an "ad hominem" against Christianity, which I could argue is not justified but in any case is not pertinent, and 2) what you're asking ultimately doesn't make sense.
We're having a discussion about what is or is not moral. Any answer you can give necessarily depends on views which can only be classified as religious, even if you are a purely a materialist; they involve the purpose of existence, what it means to be human, etc.
Consider this exchange:
A: "We should not commit genocide."
A: "Because protecting life is a universal human value."
B: "It's not universal if I don't agree with it. Why should I care?"
A: "Because we can't survive as a species unless we protect one another."
B: "What if I don't care about the species, but only about myself?"
A: "Genocide is still wrong. You shouldn't do it."
B: "Says who?"
Ultimately A has to answer "there is a larger moral principle outside of you which, whether you agree with it or not, you are obligated to obey", which is a religious statement.
The only other option is "many of us prefer that you don't do this and we will use force to stop you." That's pragmatic, but it's not about morality at all; it could just as well be applied to playing the bagpipes.
You can't exclude religion from moral debates because morality is inherently about religion.
Existence doesn't have a purpose. Existence simply "is". Purpose (in general) is the goals of pre-planned actions taken by intelligent agents acting to achieve specific ends through whatever agencies they believe themselves to control.
My answer to what is moral is to undertake actions that work to achieve those ends, as opposed to random junk and hoping for the best. Moral is to think, not accept dogma. For me, that is the exact opposite of religion.
I don't know precisely where we get morals from without religion. Clearly it can't be as simple as what the majority prefers: the phrase the tyranny of the majority refers to the problem with that.
We have a deep seated sense of fairness, even without religion. A recent experiment put two monkeys within sight of each other. One did a task and was given a piece of food, then the other did the same task and got a much tastier piece of food. The first does the task again, and again gets the low-value food, but this time he threw it back at the experimenter and beat the bars of the cage, trying to reach the high-value food. That sense seems likely to be where our concept of morality comes from.
Any relevant principles can be talked about without a religious carrier signal; including it only serves to alienate those who do not share the same beliefs.
Person B is not fundamentally immoral, only by the standards of a society and if his moral framework is incompatible with his society then they will leave or be terminated.