Let's say I have a god in the battlefield for more than one turn. And this turn I summon a creature that "completes" my devotion to the god. Can the god attack this turn? or it is affected by the summon sickness?
The god can attack the turn it becomes a creature, as long as you had it for more than 1 turn like you said. Summoning sickness only affects a creature if it entered the battlefield since the beginning of your turn, and it did not enter the battlefield upon meeting its devotion requirement, it simply changed to a creature. This won't trigger "when enters the battlefield" triggers either.
The god creature HAS been under your control since the beginning of your turn; it just wasn't a creature before.
Another user submitted a correct answer that is easy to read and I just wanted to provide a bunch of examples of when it might be relevant. Everything that has entered the battlefield since the beginning of your most recent turn has summoning sickness. That includes, but is not limited to:
Even creatures with haste have summoning sickness. If the creature somehow lost haste, summoning sickness would again affect your creature.
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