Before starting this topic, let me say this: I know that atheists may say the opposite is true. Although this thread is not to discuss its topic in relation to religion, let me only give this example: during the golden age of the Islamic empire, despite what many people may think, the majority of the citizens of that empire were non-Muslims!
The Christians, the Jews, and the Zoroastrians, were all allowed to keep their lands in their hands and keep their places of worship, and were not enforced convert to the new religion. The tax (Jizya) they were paying was marginal compared to the hefty taxes they were used to pay to the Roman and Pertain empires. Indeed that is why many of the citizens of those empires have welcomed and even supported the newcomers. And if it was not for this support, the small number of Muslim Bedouins and other Arabs with their ill equipped forces and poor strategies would not have been able to defeat the two great world powers of that time.
Now, let us return to our main topic, there is a real inherent ethical problem with atheists. They have no red lines coming from an outside source, such as religion. So they follow their own minds, While we all know that the human minds are limited. and humans are weak in nature, and their decisions may influenced by many factors. Even the opinions of the masses may be misguided by some evil powers.
What do you think?!
I don't think it is really any different for religious folks in that they are also only following their own minds. In the case of believers, they only assume their morality comes from an outside source.
You have red lines, we have read line. We just see them as coming from the same source.