If someone wondering how big is the Megami Tensei universe (the series as a whole), this picture below probably the closest I can get. There are other pictures too but I guess this one is the one that have connection between the games.
EDIT: I remove the image link just to avoid the headache of the lore and consistencies.
Imo what I like from this series (especially on the main line series) compared to the other JRPG games is that the mythology portrayal, the mature themes and the battle of "Law vs Chaos" is that great and doesn't shy away from source material. I couldn't explained it myself but it is just something different than your average JRPG.
Read at your own risk
Literally in one of the mainline game, you have to battle YHVH
The social element is introduced in Persona 3 however the school setting can be traced back to SMT: if... that also introduces the spin off series Persona (Persona 3 before the re-release P3FES was marketed with Shin Megami Tensei moniker).
Now that you mention it, also no Synchronicity prologue, SH2, IV:A missing... Definitely this chart came before Persona 5 as it has the placeholder art instead of the released game cover art.
No, however it loosely contain some elements of mainline games like alignment "Law vs Chaos", multiple endings, and moral/motives. Full body expands the multiple endings a bit especially with the new character.
Funnily the Persona series deals topic that is closer to our day to day live (i.e Persona 3, Persona 4) compared to mainline/other megaten spin off series.
As grim as it gets...
In Persona 2 you have to battle against Nyarlathotep disguised as Fuhrer.
I consider myself averagely informed in games in general, and yet I have absolutely no idea what the Persona series is all about. I need to find the time to research a bit out of curiosity, I promised myself that.