Megami Tensei itself encompasses tons of sub series.
There were the Famicom Megami Tensei games when Namco was the developer, then it changed hands to Atlus and we got the flagship Shin Megami Tensei series.
Devil Summoner and Devil Summoner: Soul Hackers were "what if?" scenarios that depicted an alternate reality where
the apocalypse in Shin Megami Tensei I never happened. Nowadays, the Devil Summoner games are a wholly different, and good, flavor.
Persona was an offshoot series, but there are nods to mainline SMT. Like the female protagonist in SMT If... makes cameos in P1, P2: IS, and P2:EP as Tamaki and many NPCs are students from the SMT If... high school.
Then you have the Majin Tensei and Devil Survivor SRPGs, Digital Devil Saga, Strange Journey, Devil Children (aka DemiKids), Last Bible (the first game was released as Revelations: The Demon Slayer), and a ton more. It's hard to keep track.