Symbolism, Imagery, Allegory
In the latter half of the book, Father Latour hears a legend about a secret fire that the Native Americans of New Mexico have kept burning for centuries, even though no white man has ever seen it. The narrator tells us this directly:
It was said that this people had from time immemorial kept a ceremonial fire burning in some cave in the mountain, a fire that had never been allowed to go out, and had never been revealed to white men. (4.1.13)
Again, Father Latour has to confront just how much he doesn't know about New Mexico, a place that he's supposed to make all Catholicized.
When he takes time to really think about it, Latour decides that he doesn't believe the stories about the secret fire.
Father Latour thought this hardly probable. Why should it be very arduous, in a mountain full of timber, to feed a fire so small that its whereabouts had been concealed for centuries? (4.1.13)
In other words, he doesn't really like the idea of the Native Americans sneaking around and doing all sorts of religious ceremonies behind his back. Latour likes to think that he's in control, and it's not until his friend Kit Carson confirms the fire story that he truly starts doubting his own powers of observation.