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.
May contain spoiler
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
Oh and Megami Tensei almost always has banger OST if you all never played them.