You've just said a very revealing thing. Are you telling me that the only reason you don't steal and rape and murder is that you're frightened of God? (Richard Dawkins).
That's ill-conceived in many respects:
i) The contention is rather contradictory. After all, atheists routinely assert that Biblical ethics is "hateful." If so, how is that a moral restraint on Christians?
ii) It begs the question by presuming there are wrong things Christians would do unless their theology restrained them. But, of course, an atheist is not entitled to stipulate moral realism in the first place, then tut-tut Christians.
iii) Lack of moral inhibition doesn't mean you want to do anything in particular. Maybe I don't think it's wrong to pirate Barry Manilow recordings. That doesn't mean I'm tempted to pirate Barry Manilow recordings. I've never had the slightest inclination to listen to Manilow. Even if I could do so with impunity, I wouldn't.
iv) Christian critics of secular ethics by no means concede that Christians would be more prone to rape, murder, and pillage than unbelievers if they lost their faith. Lifelong atheists have the same evil propensities as apostates.
v) But now I'd like to turn to my main point: the contention has it backwards. It treats Christian ethics as a kind of add-on. An artificial code of conduct that's superimposed on neutral human nature–in contrast to moral intuition or inner direction. But that mischaracterizes Christian ethics.
Oftentimes, Christian ethics liberates us to do the right thing. It is sin and society that inhibit us from doing the right thing. Christian ethics isn't so much adding moral norms, but removing impediments to moral norms. For instance, there are situations in which a person instinctively wants to do the right thing, intuitively knows the right thing to do, but his peer group or the legal system deters him.
Take people living under the thumb of a corrupt regime. Might be a police state or a banana republic. They witness widespread injustice. There are times when they'd like to intervene, but it's too dangerous. Likewise, there are times when they may be ordered (at gunpoint) to commit evil.
Or you can have peer pressure in high school or college that discourages people from "getting involved" because there's a social sanction for sticking your neck out.
To take a comparison, suppose a person has sociopathic impulses caused by brain cancer. If the brain cancer is treatable, he will lose his sociopathic impulses. The treatment didn't give him a conscience; rather, the treatment removed a barrier, thereby allowing his conscience to resurface.
To a great extent, Christian ethics gives us the courage to do the right thing, by corroborating our conscience, and by making the cost of bucking the system acceptable. Even if we are persecuted, God will ultimately reward those who obey him.
It isn't just about moral restraint, but moral freedom. To be at liberty to do good or resist pressure to do evil. Christian ethics is inhibiting with respect to vice, but liberating with respect to virtue.
When I say "instinct" or "intuition," I don't mean that in a naturalistic sense, but in a natural law sense. Absent divine creation, there is no right or wrong.
Moreover, I'm not suggesting that intuition gives us an infallible moral blueprint. Revealed norms can be a corrective. Likewise, revealed norms can resolve moral uncertainty.
But in many cases, Christian ethics isn't so much about giving us new information, but confirming the right course of action, and giving us an incentive to do the right thing. Due to common grace, many atheists retain some remnants of common decency. But that can be smothered by expediency. It isn't worth the risk. Likewise, why deny yourself?
Christian ethics is at least as much about the motivation to do right as the knowledge to do right. You can afford to do the right thing, even if that will cost you dearly, because this life is not all there is.
Only a fool would voluntarily put his head on the chopping block to save another–if there's no payoff. We need to know that God has our back.