Immorality == evil by definition. (“Evil” is meaningless without reference to a moral canon.) If the alternatives were amorality and evil, then there would be a philosophical question…
It is harder for humans to follow a moral code without a religion. Philosophies are the “Virtual Machines” of morality; they don’t necessarily define “moral” and “evil” properly, but they help our brains understand how to live according to morality and to avoid evil. Religions are simplified and managed forms of philosophy.
Aristotelianism strikes me as a sturdy, object-oriented system - sort of a “C++”. Mediaeval Catholicism made an effort to buffer that system and to provide a few additional features - call it “Java” (Orthodoxy can be “C#” in this analogy).
Atheism cuts through all of that - they’re back to coding in Assembler. You can get some impressive results from Assembler… IF you’re a genius. Unfortunately most humans aren’t geniuses. Coding in Assembler, for most of us, will crash the system.
I expect most humans, if they lose faith in God, will also lose faith in their own culture and ultimately in the future. Atheists are not good at, for instance, breeding.