[reasonisfaith]

My Christian faith is non-participation in Buddhism, for example.

It’s also non-participation in the belief that a material existence and accompanying lifestyle is the best choice.

[GunRunner]

Christianity is a religion, but non-participation/belief in Buddhism is not in and of itself a religion.

Now apply that to all religions and combine it with a non-belief in the supernatural. That in and of itself is not a religion in the dictionary sense. Much like not playing sports is not a sport itself.