Atheists do not have a moral theory. They merely criticize other people's theory. At best, atheists could have an elusive, undocumented one, but even that is debatable.
An undocumented theory can never be used to successfully attack a documented one. That is the fundamental weakness of the atheist take on morality.
Furthermore, the atheist attack on foundherentism itself is laughable. In that case, they should also reject mathematics.
Atheists inevitably find themselves before an insurmountable obstacle when trying to take on Judaism or Islam. In fact, atheists have always known that they do not stand a chance against Judaism and Islam.
In theory, Christian morality also rests on Jewish Law, just like Judaism.
In practice, however, the Christian take on Jewish Law is a patched and hacked concoction with numerous undocumented alterations. Therefore, in practice, Christian moral theory is as undocumented as the atheist one.
Concerning undocumented patching and hacking of the Law, Jesus warned his followers in his Sermon on the Mount not to do exactly that.
You can find Jesus' explicit no-patches-and-no-hacks orders in Matthew 5:
For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven. For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven.There are good reasons why the atheist adversary can trivially breach the Christian lines and easily overrun their positions. In fact, and as expected, the atheist adversary is effectively wiping the floor with them.
What else do you think that the result would be, of the Christian willful insubordination to the explicit orders of their own supreme commander?
Jews and Muslims are actually much better at carefully implementing the detailed instructions and operational guidelines issued by the headquarters of the Christian supreme command.
That is why Judaism and Islam are doing absolutely fine, while the vessels of the Christian navy, sailing on the high seas, end up, one by one, hit by a torpedo and irrevocably lost in action.