i wonder, what kind of qualities should a god have, so that atheists would start believing in that god?
As an atheist I can possibly offer a few rambling thoughts.
The key realisation would be that god does not ‘exist’. From this point onward the atheist can begin to work out the truth of the matter.
Let's examine it from a Christian POV, as I'm familiar with it. Creation in Genesis is a matter of division, spirit upon the water, light and darkness etc. This depicts a movement from a non-dual state into duality. Time, the elements etc. arise and we end up with Adam and Eve in the garden. Adam is still not sundered from the divine but he abides within it in a state of unknowing, which can't last.
The knowledge of good and evil (the fall) is nama-rupa, the mind of discrimination and self-grasping awareness which takes consciousness into grosser duality, hence the 'expulsion from the garden', which is actually an instruction manual for the return journey.
The junction between night and day (when the flaming sword of the guardian Cherubim is extinguished) is the best time to enter samhadi - look at the colour of Shiva's skin. Clues everywhere.
In this duality of 'existents' there can be no god, as the supreme cannot be limited by any distinctions. Like reflections in a mirror which do not taint the mirror, existents are illusory and transitory but are possible only because they are held in the mirror (existence/god/awareness).
So, if you meet god on the road you might not want to kill him but certainly resist the urge to believe in him.
I don't imagine that would appease any theist though.