In a letter to Thomas Jefferson, John Adams wrote

Twenty times in the course of my late reading have I been on the point of breaking out, “This would be the best of all possible worlds, if there were no religion in it!!!”

Positive Atheism tells us

This was not his reasoned opinion. Although John Adams often felt an urge to advocate atheism as a popular world view (because of the sheer abuses perpetrated by religious charlatans), he was of the firm and reasoned opinion (basically undisputed in his day) that religion is essential to the goal of keeping the masses in line.

Clearly, as Adams used the term, ‘atheism’ means “no religion”.

Jefferson famously said that

religion is a matter which lies solely between man and his God

What of those who have no God? They have no religion. Jefferson also said

Because religious belief, or non-belief, is such an important part of every person’s life, freedom of religion affects every individual.

Atheism is the belief that there is no God. It implies non-belief that there is a God. Atheism, as understood by Jefferson, is not a religious belief.

John Lennon wrote a song in which he famously asks us to

Imagine there’s no heaven
It’s easy if you try
No hell below us
Above us only sky
Imagine all the people living for today

Imagine there’s no countries
It isn’t hard to do
Nothing to kill or die for
And no religion too
Imagine all the people living life in peace

By religion, Lennon means belief systems which include beliefs in the supernatural—God, Heaven, Hell, etc. His view is that we’d be better off in a world in which there was no God, i.e., one in which atheism was true. In such a world there would be “no religion” and “all the people living life in peace.”

Atheism, as understood by Lennon, is not a religion.

Once every 5 years, New Zealand citizens are required to participate in a census. One of the questions seeks to elicit the respondent’s “religious affiliation”. According to Wikipedia,

In the 2006 Census, 55.6 percent of the population identified themselves as Christians, while another 34.7 percent indicated that they had no religion

That’s right, there is a box to tick labeled ‘No religion’. There are no boxes to tick labeled ‘Atheist’ or ‘Humanist’ or ‘Objectivist’. There’s not even a box labeled ‘Jedi’.

Atheism, as understood by Statistics New Zealand, is not a religion.

Wikipedia’s has an entry titled Irreligion in New Zealand.

Irreligion —the absence of religious belief or affiliation— is an increasing trend in New Zealand. Although New Zealand has no established religion, Christianity has been the majority religious affiliation since European settlement in the 19th century. The trend toward irreligion may indicate increasing secularisation as well as a rise in non-institutional spiritual belief.

Atheism, as understood by (some) Wikipedia authors, is not a religion. It’s an irreligion!

The meaning (or meanings) of a word is determined by the conventions governing its use. What those conventions are is determined by the facts of usage. (See examples above.)

It is not conventional to call atheism a religion.

It is conventional to rebuke those who do. 😉