We use the word dogma to mean two different things, I will only talk about the use of this term in philosophy.

In epistemology, Dogmatism is the opposite of skepticism. Skepticism is to say that knowledge (or at least some kind of knowledge) is impossible in principle.

Dogmatism on the other hand is the claim that some knowledge (or all knowledge) is possible, in principle, even if we do not currently have such knowledge. Dogmatism (dogma in general, in philosophy or day-to-day use) can be described as a claim, a positive statement about knowledge.

Now, what is atheism?

Atheism is the absence of belief in one or more deities. An atheist does not claim nor deny the existence of God.

Since not every atheist claims that God does not exist, it then follows that "denying God's existence" is not the appropriate definition of atheism.

Suppose I claim that it is possible to know whether you wear a red shirt now. This is dogmatism , and if I claim that it is impossible to know, this is skepticism.

And if I say that you wear a red shirt , and I know it, and nothing can change this, this is dogma as the term is more commonly used in daily our daily life.

Atheism on the other hand is a very different subject. Three kinds of people I can think of now : Those who believe that you wear a red shirt, those who believe that you do not wear a red shirt, and those who believe neither, they just suspend judgement.

Which ones are a-redshirt-ists here?

The ones that believe you wear a red shirt are believers.

The ones that don't are all a-redshirt-ists including those that just do not believe.

If I say that I am an a-redshirt-ist, it does not mean that I believe you wear another color (blue, green or yellow..etc) , it just means that I do not have any good reason to believe that you believe a red shirt.

This is not a claim (remember, we said that dogma is a claim).

It is contradictory to have one thing be a claim and not be a claim at the same time, therefore : Atheism is not a dogma.

So, this is not a claim that I have knowledge or that knowledge is possible as covered by the philosophical and common definitions of the word dogma.

We use the word dogma to mean two different things, I will only talk about the use of this term in philosophy.

In epistemology, Dogmatism is the opposite of skepticism. Skepticism is to say that knowledge (or at least some kind of knowledge) is impossible in principle.

Dogmatism on the other hand is the claim that some knowledge (or all knowledge) is possible, in principle, even if we do not currently have such knowledge. Dogmatism (dogma in general, in philosophy or day-to-day use) can be described as a claim, a positive statement about knowledge.

Now, what is atheism?

Atheism is the absence of belief in one or more deities. An atheist does not claim nor deny the existence of God.

Since not every atheist claims that God does not exist, it then follows that "denying God's existence" is not the appropriate definition of atheism.

Suppose I claim that it is possible to know whether you wear a red shirt now. This is dogmatism , and if I claim that it is impossible to know, this is skepticism.

And if I say that you wear a red shirt , and I know it, and nothing can change this, this is dogma as the term is more commonly used in daily our daily life.

Atheism on the other hand is a very different subject. Three kinds of people I can think of now : Those who believe that you wear a red shirt, those who believe that you do not wear a red shirt, and those who believe neither, they just suspend judgement.

Which ones are a-redshirt-ists here?

The ones that believe you wear a red shirt are believers.

The ones that don't are all a-redshirt-ists including those that just do not believe.

If I say that I am an a-redshirt-ist, it does not mean that I believe you wear another color (blue, green or yellow..etc) , it just means that I do not have any good reason to believe that you wear a red shirt.

This is not a claim (remember, we said that dogma is a claim).

It is contradictory to have one thing be a claim and not be a claim at the same time, therefore : Atheism is not a dogma.

So, this is not a claim that I have knowledge or that knowledge is possible as covered by the philosophical and common definitions of the word dogma.