What's the difference between narcissistic self-aggrandizement and pantheism?

In philosophical (Platonic) life, piety is practicing philosophy whose aim is to "become as godlike as possible" = "serving one's own God", i.e., one's own self.

On a personal level, piety is being good to one's own self, the inner divine intelligence, by recognizing its divine identity and acting according to what is good for the self (nous) in Platonic terms.