By swearing that I do not believe in the literal existence of any deities.
Is there any other proper way?
Fair enough.
We have of course had various answers along time according to the level of scientific knowledge... but ultimately it does not (or should not) matter from a religious standpoint. This is not a question of interest to religion, far as I understand it.
Perhaps a few. It does not seem to be trying to, going by the text itself.
Among other reasons, because it disapproves of Atheism just because, of course.
A slightly more rigorous analysis shows also that it is technically flawed, since it encourages people to take it as a perfect, and unchanging reference for religious purposes. That amounts to a call for ignoring the reality of societal change and anthropological variety.
Because I looked around.
It helps that when I learned of the concept it was from sources that taught specific versions of it that almost seem to have been designed to be disbelieved, I suppose.
Really? Why would you?
More to the point, what other proper justification could either exist or be called for? None occur to me right now.
I became an atheist by being born and eventually learning of the concept of God and realizing that it was not for me.
Truth be told, I don't think it really fits in a healthy way with a very many people either. Perhaps 10% of people actually make a good match with the concept. Others basically pretend to believe or just don't care to say that they do not. Far too many are enabled in various kinds of derangements by it.