The ones who call atheism a religion are the ones who have no idea what atheism is or what a religion is for that matter. Unless it falls into their own beliefs and practices, they are oblivious. So, you will be hard pressed to find someone who calls atheism a religion capable of explaining it as such without reflecting in some way on their own attitudes about their own religion and beliefs.
Religion:
1. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
1.(A) A personal or institutionalized system grounded in such belief and worship.
2. The life or condition of a person in a religious order.
3. A set of beliefs, values, and practices based on the teachings of a spiritual leader.
4. A cause, principle, or activity pursued with zeal or conscientious devotion.
None of these definitions found describe atheism. But, trying to explain that to them is like trying to explain thermodynamics to a two year old.
Religion:
1. Belief in and reverence for a supernatural power or powers regarded as creator and governor of the universe.
1.(A) A personal or institutionalized system grounded in such belief and worship.
2. The life or condition of a person in a religious order.
3. A set of beliefs, values, and practices based on the teachings of a spiritual leader.
4. A cause, principle, or activity pursued with zeal or conscientious devotion.
None of these definitions found describe atheism. But, trying to explain that to them is like trying to explain thermodynamics to a two year old.