"Your faith is blind. You're a fanatic."
The irony of this line is insane.
(And people complain about the writing in this film...)
When Sator says this, he's talking down to The Protagonist, mocking The Protagonist for being committed to a cause he doesn't truly understand. The irony is that this doesn't actually apply to The Protagonist — it applies to Sator.
When Sator says it, they are both technically acting on blind faith, since neither of them can know for certain who they are serving.
By the end of the film though, we know that The Protagonist is actually acting on his own instructions — instructions that come from a future version of himself that has already lived these events. Therefore, he's not acting on blind faith at all, but following his own guidance — guidance informed by his own first-hand experience.
In contrast, Sator is being guided by whoever sends him the gold in the capsule...
(...whether that's the future Antagonists trying to acquire the Algorithm or The Protagonist secretly pulling the strings...)