[@Expolar], just to help you out. 
Natsu first went Dragon Force against Jellal after eating Etherion. He goes Dragon Force again in his fight again Brain/Zero(I think that was the name) after eating Jellal's Flame of Rebuke. 

His next significant power up was eating Laxus's Lightning (Which Laxus GAVE him <---- I felt this is important)to enter Lightning Flame Dragon Mode, hence the dual element part. 

While Natsu and Gajeel can enter their dual element mode at will, it is very taxing and could leave them very tired afterwards, if they are not used to it. 

As per the manga and information given. A first generation Dragon Slayer can can enter Dragon Force only by consuming and high level of magic in the form of their element. A second Generation Dragon Slayer is considered to be in a perpetual constant sort for Dragon Force, though it is not really the same. A third generation Dragon Slayer can enter it at will, though the power has not been shown to be on par and was rather lack luster.

As for entering it at will, without giving any major spoilers:
[hider=Just in case this is considered major] Natsu is the only Dragon Slayer who has been shown to enter Dragon Force at will. I will not tell you how and why, but being that it like just happened not too many chapters ago, we can't say for sure whether or not he'll be able to do it again.[/hider]

Essentially, I think most of this information could be interchangeable in switching the words Dragon with God. I don't know if that helps you or Hata in anyway, but I figured the information was pretty well organized.