How did Saitama become so strong?

 Saitama of One Punch Man is arguably the strongest anime character, who can end a battle with anyone with just one punch and is among the most popular anime characters known to any normal anime fan! As a fan of Saitama and his enormous amount of strength, I have always wondered how he got this strong...

So I decided to dig a little deeper into the One Punch Man manga and webcomic to connect the dots of different popular theories and decrease the gap between those theories to try and provide you with an answer to the question," Why is Saitama so strong?"

According to Saitama, he became this strong because of his rigorous workout routine consisting of 100 push-ups, 100 sit-ups, 100 squats and 10 kilometers run. Obviously no one believed this, and what's more? Genos who has never spoken to Saitama raising his voice, for the first time, told his sensei to cut the crap and get serious. And in my opinion, although it was never mentioned anywhere, I think even the Mumen rider must be having a more rigorous training routine than this one, but is no where near as strong as Saitama. So that cannot just be the answer to our question.

Now there is a theory I like a lot. "In the OPM world, your thoughts, beliefs, obsessions or any such extreme mental state blend into reality. For example, Crablante became a crab monster because he was obsessed with eating crabs. With Saitama, the same thing happened except his mind wasn't corrupted. He wanted to be a superhero who could beat villains with one punch so he trained his ass off everyday and fought with monsters during his 10km run and escaped death frequently. His will to become a hero was absolute and with 0 evil intention so at the end his mind wasn't corrupted. This routine started breaking his limiter overtime because he truly believed in becoming a superhero and exercised with only one goal in mind. Henceforth, his thoughts manifested into reality through his exercise routine which is why he started gaining strength and eventually when his limiter completely broke he harnessed infinite power."

This theory somehow feels more acceptable to me than the "rigorous" training Saitama mentioned. There are multiple characters who experienced something similar in the opmverse, Garou, who wanted to become a monster, at last has become a monster.

One more theory, that has been coming to shore is that some GOD is the source of Saitama's power. OPM universe is bound to have multiple gods and maybe one of them is the reason for Saitama's superhuman strength!

What are your theories about Saitama's incredible strength?