The only reasons I can think of for denying that atheism is a religion are ignorance and dishonesty. It has been defined as a religion by courts, definition, and the actions of professing atheists. They demanded their own chaplains for the military, and Harvard hired Satan himself — oh, wait, that is a different story. Harvard hired an atheist as chaplain.
One of several schools that was founded on Christian principles but jumped the fence into apostasy, Harvard hired this “chaplain” pretends to be “good without God” and claims to be able to provide spiritual guidance to everyone. That’ll be the day! It has been clearly demonstrated that professing atheists are contumelious toward Christians, especially biblical creationists. They define “good” in a postmodern, relativistic way that fits the culture of the moment; whoever betrayed Jews to the Nazis was doing a good thing by Nazi standards.
God calls people like this fools, and anyone going to an atheist “chaplain” is asking for spiritual disaster. Indeed, there are marked similarities between atheism and the LaVey brand of Satanism. Atheism is irrational, lacking the necessary preconditions of human experience. It also has no consistent moral foundation, as is painfully obvious.
Don’t be disunderstanding me here. Yes, there are atheists who have high moral standards, and may even have better conduct than some “Christians”. But as with science, logic, consciousness, love, goodness, and other things, they cannot give a coherent justification for morality. They believe everything came from nothing, yet mock the biblical Christian worldview, which is the only consistent and rational expression for human experience.
Goodness is not relative. Atheists, like other unbelievers, need to humble themselves and repent, making Jesus Christ the Lord of their lives.