They are not the God of an era because they don't want it or deserve it or their law/desire are pretty twisted.
Penemue: How?
Issei: That's because, do you remember the time god that I mention before?
Gabriel: Yes, you did mentioned him.....no way you don't mean...
Issei: Yes, he is also an Hegemony God, but he doesn't want to take the throne due to a reason.
Penemue: And that reason was?
Issei: Because his law originates from his desire of "I want to enjoy this moment forever", to experience his daily life for all of eternity. When activated, it completely stops time in all levels of existence. And who want to govern the existence where time doesn't even exist. Do you understand a world where time doesn't even flow, and that's how he earn the title: The Eternal moment.
Penemue: I see, Then I know why the other one known as The Gold can't be a ruler.
Issei: And that is
Penemue: That's because his desire consist of eternal war and conflict.
Issei: Indeed you're correct. Reinhard's law originates from his desire of "I love everything at full power", to spread his love throughout the entire universe and opens the Gates of Valhalla, releasing the Day of Wrath. and his soldiers under him are immortal and fight for eternity.
Gabriel: I don't know how was he able to 'spread' his love, if he always spread conflict.
Issei: Like said their Law is twisted.
Reinhard & Ren: ( Oi...)
Issei: ( just being honest here even I can find you desire quite a cuckoo.)
Mercurius: ( Someone understand me, and I give my thumb up to you.)
Ren & Reinhard: ( SHUT UP!!! OLD MAN/KARL!!!!!)
.
.
Issei: hehehe....
Gabriel: Uhmm why are you laughing?
Issei: Oh that because someone is really pissed off.
Both: Oh I see.
Issei: Anyway have any questions?
Gabriel: Uhmm how to become a ruler, the one who sits on the throne.
Issei: there is 2 ways which are; Killing the current god is something that is the last thing I do and become the current heaven's successor
Penemue: So how do you become a successor.
Issei: Uhm, that because I somewhat gained their interest.
Gabriel: Uhmm I see.
Issei: Any questions.
Penemue: So how do we gain access to the Die Ewigkeit formula
Issei: lemme check.
Issei: ( Hey system, How to let them activate their Ewigkeit?)
System: [THEY CAN'T.]
Issei: ( Huh!?! WHY!!!??)
System: [INSTEAD OF AWAKENING THEIR EWIGKEIT THEY HAVE QUINTESSENCE OR EQUATION DUE TO THEIR ANGEL OR FALLEN ANGEL PHILOSOPHY.]
When Issei heard he thinks for a sec and look through his library full of memories and he realized that he read about Quintessence or Equation before in the internet.
Issei: ( I see luckily I know some of the quintessence or equation)
Issei: ok girls the thing is you can't.
Penemue: What do you mean we can't?!?! Please explain?!.
Issei: sigh.. instead of the Die ewigkeit formula, you have activated a type of formula that are able to do the same thing but it is way more complicated than the Ewigkeit formula.
Gabriel: And what does this formula called?
Issei: Is called Quintessence or you can called it Equation for short.
Penemue: So how does it work?
Issei: Ok let me explain about it; Quintessence also know as the Fifth Element, It is a higher-dimensional element that governs all basic four elements. Quintessence used by Angels, by accessing through the gate of heaven, they can draw out the fifth element from higher Sephiroth. However, due to Satanael, The 3rd Generation God control over the leylines, Angels require his permission to use Quintessence, via equation "Access—Master". And it's counterpart, Negative Quintessence, or commonly known as Original Sin or Sin of Adam known as the 2nd Generation God. Originating from the Abyss of Judecca, which exist during the third heaven. It is a wavelength inverse of the world. Demons themselves, while hostless, are made out of negative quintessence. As the general rule of the fallen paradise where evil reigns, negative quintessence is stronger than its positive counterpart and the demons' equation is "Access—My Sin".
Gabriel: Are we suppossed to worry if some devil use it.
Issei: oh you don't have to worry about it anymore because this type of equation was lost during the third era.
Penemue: Whew...so how do we able to use equation?
Issei: I honestly have no clue because I'm no angel because it is the passcode used mostly by Angels and Dust Angels which now you called it fallen angel to access and utilize Quintessence. By interfering with Gaia's ley lines which I also what the hell that is since it is a lost information, they can use the path to draw out the fifth element from higher dimensions of either Heaven or Abyss. You can can ask the Morningstar to help if you have his permi--
Before he even get to finish his explaination Nerose interrupted him
Nerose: They have my permission, but it is not the time yet.
Issei: Oh well, lucky for you girls he gave his permission.
Gabriel: Well I expected he is kind to us since he is the progenitor of angel.
Issei: I wouldn't say he's kind, more like a calculative and a realist one.
Nerose: ( I don't deny that.)
Issei: Any questions?
Gabriel: Is there's rank within the angels who use quintessence.
Issei: The ranks are:
First Triad:
• Seraph ( Tier 3) • Cherubim( Tier 2) • Throne ( Tier 1; Hegemony God Level)
Second Triad:
• Dominion ( Tier 4) • Virtues (Tier 5) • Power( Tier 6)
Third Triad:
•Principalities( Tier 7) •Archangel (Tier 8) •Angel( Tier 9)
And right now you are at the initial level of Second Triad; Power which is not bad, and difference in strength in each level is like heaven and earth, so you don't have to be rush for power although I doubt you will do that. Any more questions?
Both: No, were all good.
Issei: Okay then shall we headed to the next level?
Both: YES!!!!
And so they headed into the next floor.