A bit of a topic shift, but this does also bring up something else I had been thinking about recently.
Powerful mortals like Diana and Rin mostly have innate potential (such as from a bloodline) that they can enhance and train, much like an Olympic athlete or master martial artist has genetic potential that is made use of with training and experience. The power is innately "theirs" but they will eventually hit limits to their power they cannot surpass because the "limits" are hard, if not impossible, to shift.
However, it's been stated or implied several times that MC and gods/celestial entities are a bit different. They do have some innate powers themselves, such as MC being stronger, faster, more durable, and having a wind affinity. But a lot of their power potential appears to come from souls and from the worship/devotion of their followers. We've seen several times where celestials actually get weaker when they lose followers, for example.
MC has had big power boosts from absorbing powers and recently from artifacts, but we haven't directly seen anything like him growing due to worship. I suspect that part of this island arc is going to be related to that though. It's pretty obvious he's going to get some power boost and probably celestial knowledge from the goddess temples and whatnot, but it would not surprise me if at the end of this arc after he likely clears all the temples, restores magic to the island, and people learn he's Nephilim, some of the island inhabitants start to informally (or maybe even formally) worship and revere him, even if just in secret.
It wouldn't surprise me if a combination of absorbing a few more powers and starting to gain "followers" is what triggers the Ascension that's been mentioned as a future major power boost and potentially starts moving him from "high tier mortal" to "minor god".
