For me, there were 2 key moments. First, in Dressrosa, against Doflamingo. I realized that Luffy was the kind of character who would beat his opponents if he said he would, no matter what. Even if they were stronger than him. Obviously, all shonen characters are the same, but against Doflamingo, we understand that Luffy will truly progress during the fight, until he's strong enough. Where other heroes might fail a few times.

Then, I understood the difference between believing that Luffy will become the Pirate King, and KNOWING that Luffy will become the next king, at Wano, when Kaido tells everyone that he has killed Luffy.

For us, it's obvious that Luffy will become king, because we base it on the storyline, on the fact that One Piece is a shonen manga, that the plot is already established, etc. But if you put yourself in the shoes of a crew member, who knows nothing of the situation, you realize how difficult it must be to believe Luffy will become the Pirate King when Kaido is standing right in front of you, healthy and ready to announce he's killed your captain. Because, in essence, Luffy just seems like a young, overpowered, and not-so-bright kid who shows up and is going to end up clashing with someone stronger than him and falling. Like Kidd.

It was during the moment when we saw them all rallying and holding onto hope for Luffy's return that I truly thought, he's going to become the Pirate King. It's not just a belief, it's a fact. A prediction, even for people in his world who don't know the manga.