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.
302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. A creature can’t attack unless it has been under its controller’s control continuously since his or her most recent turn began. This rule is informally called the “summoning sickness” rule.
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:
- If you play Mutavault and then use its second ability, it will not be able to attack that turn unless it somehow gains haste.
- Oddly enough, if you play Mutavault and then turn it into a creature, it becomes affected by summoning sickness and can no longer use its first ability to tap for mana. It always had summoning sickness, but only became affected by it when it became a creature.
- If you play Gideon, Champion of Justice and then activate its second ability, it cannot attack. That's actually a ruling on the card if you click the link.
- If you play Rakdos Keyrune and immediately activate it, you'll no longer be able to tap it for mana, and you won't be able to attack with it either.
Even creatures with haste have summoning sickness. If the creature somehow lost haste, summoning sickness would again affect your creature.