That's not necessarily true. Lots of agnostics and atheists are still riding on the cultural momentum of a used-to-be-Christian society. They derive their best moral principles from th Judeo-Christian tradition, whether they acknowledge it or not.
Even the deist/agnostics among our Founding Fathers (e.g. Jefferson and Franklin) had cultural assumptions which were Judeo-Christian --- like this notion they considered "self-evident" that people are "created" equal. That can be true only with religious and transcendent assumptions --- whether acknowledged or not.
Atheists are not so bereft of morals as you think. St. Paul said they still have the moral law in the hearts, and it approves them when they do right, accuses them when they do wrong.
Believe what you want. Whatever helps you sleep.
>> Atheists are not so bereft of morals
The modern atheistic movement is primarily a rejection of Christianity despite the parallels that might exist in practice.