My local playgroup really needs to ban the use of Humility.

There's no argument that a God that hasn't met its devotion requirement to become a creature keeps its abilities. However, my playgroup in a recent Commander game got caught up in a discussion as to whether or not a God that had become a creature would ever revert back to being a noncreature. The argument was made that when it becomes a creature, it loses the ability that makes it not a creature if you don't have enough devotion, and thus it stays a 1/1 with no abilities until Humility leaves the battlefield even if the player drops below the required devotion. Someone else said that the ability that made it not a creature would still function if the God had been on the battlefield before the Humility landed. Which would seem to hold up going by the Layer Rule, but Humility honestly gives me such a headache that I'm not sure how anything really works with it (except Humility + Opalescence, because that interaction is worth memorizing and would seem to also support the "God turns back into noncreature" argument).

Can anybody set straight what happens?

If Heliod, God of the Sun and Humility are on the battlefield, and the player doesn't have enough devotion to white, we have this:
Layer 4: Heliod stops being a creature.
Layer 6: Humility doesn't affect Heliod since Heliod isn't a creature.
Layer 7b: Humility doesn't affect Heliod since Heliod isn't a creature.

If Heliod, God of the Sun and Humility are on the battlefield, and the player does have enough devotion to white, we have this:
Layer 4: Nothing happens, since Heliod's condition isn't met.
Layer 6: Heliod loses all abilities.
Layer 7b: Heliod becomes 1/1.

Those two situations are the only possibilities. Since the effects are all in different layers, timestamps don't matter.

(Well, if there are also other creatures involved, the relative timestamps would matter for whether they have vigilance or not, since both Heliod's vigilance-granting and Humility's ability-removing are in layer 6. So in the situation where the player doesn't have enough devotion to white but does control other creatures, those other creatures will have vigilance if and only if Heliod entered the battlefield after the Humility.)

The layer system is really the key here. At every point in the game, to determine the game state, you go through all the continuous effects and apply them in layer order. So for a layer 4 effect, you have to consider the base characteristics of the objects and any changes from layers 1-3, but you don't get to peek ahead at what layers 5-7 will do and you don't consider what the game state was like at some previous time.

That's only if the player's devotion to white is less than five. Otherwise Heliod's vigilance-granting effect depends on Humility's ability-removing effect and nobody has vigilance, no matter the timestamp order.

Good catch. Corrected.