you mean in the mental realm ?
irl he isnt as strong as he used to be but he probably the 2nd strongest in the world. honestly this makes me wonder just how crazy strong he used to be.
in the mental realm, from my understanding, when mc perceives te opponent as weak or he is stronger than them, then he will over power them.
To be fair, most of his strength came from his technique and battle experience, not really leveling related or skill related like most other manhwas yknow
He was called martial god and his old body wasn't really the best for magic so most of his techniques are martial arts related. Not saying that he didn't use magic/mana since he did say he used a cheap mana breathing technique(iirc?) and learned it to the max.
He's still strong now cuz he has been working out everyday to build his stamina so his body can handle doing martial arts. And his experience and memories are still intact, so that explains it. His body now is better suited for magic and he got a better mana technique so now he only need to keep absorbing magic until he can be as strong as his prime :D
It's a bit ridiculous how the MC beats all his opponents easily and flawlessly even though he's supposed to be weaker than he originally was. He should at least face hardship once right?