How does Starfield of Nyx interact with the Theros Gods?
The general idea is that each ability is either "on" or "off" depending on whether its "as long as" clause is satisfied, so we just apply the abilities that are "on" to Heliod's base characteristics.

Situation 1. (Devotion &lt; 5, enchantments &lt; 5.)

In this situation, Heliod's "as long as" condition is true and Starfield's is false, so the only relevant active effect from those cards is


  Heliod isn't a creature.


So, Heliod is just a White Enchantment.

Situation 2. (Devotion &lt; 5, enchantments ≥ 5.)

In this situation, both "as long as" conditions are true, so the following effects are active for Heliod:


  Heliod isn't a creature


and


  [Heliod] is a creature in addition to its other types and has base power and base toughness equal to its converted mana cost.


This is the only one of these situations in which timestamp matters.

Both relevant abilities here are static abilities that generate continuous effects. Rule 613.6a says


  A continuous effect generated by a static ability has the same timestamp as the object the static ability is on, or the timestamp of the effect that created the ability, whichever is later.


So, the order in which we apply these effects depends on the order in which the permanents entered the battlefield.

Heliod enters first

In this case, we apply Heliod's effect first. So we apply the effects as follows:


In Layer 3, Heliod's ability makes it not a creature, then Starfield's ability makes it a creature. The result is that Heliod is a creature.
In Layer 7b, Starfields' ability sets Heliod's base power and toughness to 4.


So,  Heliod is a White 4/4 God Enchantment Creature.

Starfield enters first

In this case, we apply Starfield's effect first. So we apply the effects as follows:


In Layer 3, Starfield's ability makes Heliod a creature, then Heliod's ability makes it not a creature. The result is that Heliod is not a creature.
In Layer 7b, Starfield's ability sets Heliod's base power and toughness to 4, but that doesn't matter because Heliod is not a creature.


So, Heliod is again just a White Enchantment.

Situation 3. (Devotion ≥ 5, enchantments &lt; 5.)

In this situation, neither "as long as" condition is true. So, Heliod has its base characteristics, and it is a 5/6 White God Enchantment Creature.

Situation 4. (Devotion ≥ 5, enchantments ≥ 5.)

In this situation, only Starfield's "as long as" condition is true, so the only active effect on Heliod is


  [Heliod] is a creature in addition to its other types and has base power and base toughness equal to its converted mana cost.


So, Starfield redundantly makes Heliod a Creature and sets its base power and toughness to 4. The end result is that Heliod is a 4/4 White God Enchantment Creature.



As a side note, if you replace Starfield of Nyx with Opalescence, situations 2 and 4 apply in exactly the same way. If you replace Heliod with a regular (non-God) enchantment creature, situations 3 and 4 apply in exactly the same way.