what are we supposed to say again when christians ask us where we get our morality?

[Thumpalumpacus]

No, I am saying that in a purely material universe it is impossible to define any act as good or evil.

Given the fact that humans are discussing an abstraction, your premise is obviously nullified.

The evidence? You were claiming that I do not have access to God’s revealed word but the Bible is readily available to anyone who is interested in reading it so obviously that claim is easily refuted.

Which version of the Bible? Which translation?

Killing is only immoral when it violates God’s decreed will. The example you gave did not violate God’s decreed will.

Don't look now, but you're practicing moral subjectivity.

Because morality as a normative system derives from God’s character, therefore it’d be impossible for something to be morally good and yet contrary to God’s decree because God’s decrees derive from His character.

I have emphasized your bald claim which is bereft of reasonable support.

"Reason."

You should not need a cheatsheet for that.

How does reason demonstrate that the atheist can have a defensible definition of morality that is consistent with his atheism?

By using the Golden Rule and understanding that morality is both relative and subjective. It also requires empathy.

Animals kill, feed their young, wage war, practice self-sacrifice, rape, steal, commit infanticide, and so on. How do you know which of these behaviors are morally good and which ones are morally wrong?

Applying morality to animals which may not have the ability to perform the abstract thought required to arrive at morality doesn't seem very useful, to me, except as a rhetorical device.

I trust my own sense of morality over that described in the Bible, myself. I'm sure you trust your own sense of morality over it, as well.