My argument is that for anyone to self-identify as an atheist, they absolutely have some idea of what that word means when they say they are an atheist in contrast to a theist, or believer in that idea of God. It makes no sense at all for me to say I don't believe unicorns are real, if I've never heard of a unicorn before, does it?So if we go for a second with your theory that atheism is all just disbelief, which I don't believe, how then do I define God so as to not believe in him? Even if we talk the Abrahamic God, there seems to exist some debate of specific points.
Why is that insincere? It's technically accurate. In my experience, when I press self-declared atheists about their idea of what God is, after they go through a series of hemmings and hawking and avoidances, when pressed sufficiently enough, invariably they all have the Western mythic-literal traditionalist view of God as an "entity" or being external and transcendent to creation, wholly outside of ourselves and the world.It seems insincere to say "I don't believe in the Christian God", because then people will say "How about this Hindu god? Or how about...."
What is not technically accurate, nor sincere, is to declare as a defense against that question of "what about the Eastern views of the Divine?", is to then say "I mean all gods and all ideas of gods", which is what I've heard time and again in defiance of that question. Then when asked if they understand what those views are, it still comes back to their mythic-literal Abrahamic version.
So how then is it sincere to say someone rejects all ideas of God, when they don't know what they are? As a highly rational, critical thinking person, I see that as irrational and insincere. The correct response to not knowing about them would be to say, "I really can't say. I don't know if I believe or lack a belief in them. I might believe in them already, if I understood what you meant by them". Not this, "I mean all gods, everywhere", and then declare that atheism means nothing more than just a lack of belief as an excuse.
Well, that becomes a much more interesting discussion then. That's where the fun begins. Now we find find common frames of reference and take it from there.And even if we agree on everything regarding which God to talk about, who is to say you and I don't have different mental images of him almost completely?