Unbelievers sometimes taunt Christians by asking whether they'd kill their own child if God commanded them to do so. That's a popular atheist trope.
i) One problem with the challenge is the issue of coherence. We could turn the challenge back on the atheist. What would you do if God told you to do that?
You used to be an atheist until God told you to do it. Now that you rudely discover that God really exists, what do you do?
So what's the nature of the hypothetical? Does it stipulate a situation in which God actually commands you do to that? You hear an audible voice without any visible source. Perhaps a divine sign to corroborate divine authorization.
Is the hypothetical asking what you would or should do assuming that's a realistic scenario? If so, that's a dilemma for anyone, believer and unbeliever alike.
ii) Or is the hypothetical designed to test the veridicality of divine commands? If that happened to you, should you question your sanity?
But that doesn't single out Christians. Both believers and unbelievers can suffer from psychosis. Some unbelievers hear voices. So that's a conundrum for anyone.
iii) The challenge is intended to make Christians squirm. But if you think about it, the challenge is a glib, unintelligent quandry (see above).
iv) The challenge typically alludes to the "sacrifice" of Isaac (Gen 22). But that actually backfires. After all, God didn't make Abraham go through with it. So if we're using that example as the frame of reference, then God doesn't intend for you to obey that command. Divine precedent should lead you to discount the sincerity of the command.
v) Moreover, we're not in the same position as Abraham. He was a strategic figure in the formative stages of redemptive history. But we're living in the age of fulfillments–past and future. The groundwork has been laid.