I like that lots of circumstantial evidence reinforces Wakka's belief in Yevon. Chappu didn't pray at the monument the day he left. He chose to use a machina weapon, instead of the sword his brother gave him. It gives Wakka ground to stand on, too. Some people may see him as the token evangelist, but I don't think Wakka is blindly following Yevon. He may or may not see the flaws and contradictions, but regardless he rejects them because he has to. Wakka believes wholeheartedly in Yevon not because he is supposed to, but because he needs to.

I don't think we learn the entirety of Wakka's backstory until FFX-2 so I will spoiler it for the moment: (SPOILER)Wakka's parents are killed by Sin, and neither he nor Chappu has any memory of them. Wakka raises Chappu as best he can, presumably with Lulu's help as the three of them are all orphans. That is a ridiculous level of pressure to play on a young child. He watches Chappu grow into a man arguably better than he is, and then watches as Chappu turns his back on Yevon and is killed. Wakka needs to believe that Sin can be beaten, that Yevon's ban on machina is necessary, and that true faith has the potential to save people. He rejects Operation Mi'ihen because, if it works, everything he clings to for comfort is false. If it works, if Yevon is false, then his brother died for naught. The person closest to him was just taken without purpose.

Despite his positive demeanour, I highly suspect that Wakka is still deep in the mourning process for the entirety of the game. It is why he distracts himself with Blitzball, and then why he gives up Blitzball to become a Guardian: he wants to avenge his brother so that he can find peace. It also explains why he is so staunch and struggles to see the truth of Yevon, even when the other Yevonites like Lulu and even Yuna -- a Yevonite summoner! -- are able to accept the truth.

It is strange that Tidus is given further opportunities to pray or not; player choice in these circumstances is hard to reconcile with the canonical portrayal of Tidus as completely incompetent and downright blasphemous when it comes to Yevon's teachings and practices. I have made him pray at every opportunity, because the situations are largely centred around Wakka, and I feel like the Tidus-Wakka relationship would prompt Tidus to support his (new) friend.