As Father Vaillant remarked, at Rome they did not seem to realize that it was no easy matter for two missionaries on horseback to keep up with the march of history. (7.1.2)
Father Vaillant often feels like the religious leaders back in Rome take his (and Father Latour's) efforts in New Mexico for granted. They are out of touch with how difficult it is to do religious work in this part of the world.
He attended to his correspondence, went on his rounds among the parish priests, held services at missions that were without pastors, superintended the building of the addition to the Sisters' school: but his heart was not in these things. (7.2.1)
Sometimes, Father Latour gets really down on himself and starts to wonder if there's any point to all the religious work he's doing in New Mexico. During these times, he feels completely disconnected from the world and all the people in it. But luckily, he snaps out of his depression when he sees an enslaved Mexican woman sneak away from her masters just to pray inside his church. The woman's sense of duty totally reinvigorates him.
These poor Mexicans, he reflected, were not the first to pour out their love in this simple fashion. (8.3.19)
While living in New Mexico, Father Latour learns that the Mexican people are extremely devoted to images of Mary. He reminds himself that many people throughout history have felt this same devotion to the Holy Mother, including great artists like Raphael and Titian.
Once again among his own people, as he still called them, Father Joseph opened his campaign, and the poor Mexicans began taking dollars out of their shirts and boots […] to pay for windows in the Denver church. (8.3.26)
The miners and prospectors in Colorado have plenty of money, but none of them is willing to pay a penny for the building of a church. Meanwhile, the Mexicans in New Mexico are totally poor, but more than willing to spend every extra cent they have to help build a church in a far-off place. This is Cather's not-so-subtle way of saying that Mexicans have a much stronger sense of religious duty than greedy white Americans.
[The] Bishop used to remind them that no man could know what triumphs of faith had happened there, where one white man met torture and death alone among so many infidels, or what visions and revelations God may have granted to soften that brutal end. (9.4.5)
Father Latour knows that he has taken a difficult job by becoming the Bishop of New Mexico. But he knows he's not nearly as devoted or brave as the very first white priests who came to this area and who were tortured and killed for trying to spread the Catholic faith.
"What shall I do, Jean? Help me! […] I cannot break my father's heart, and I cannot break the vow I have made to Heaven. I had rather die than do either." (9.5.6)
As a young man, Father Vaillant has an extremely difficult time choosing between his duty to his father and his duty to the church. In the end, he chooses the church and seems to be happy with his decision. But right up until the moment he dies, Father Latour wonders whether Vaillant ever regretted his decision.
Carson was a soldier under orders, and he did a soldier's brutal work. (9.7.3)
Kit Carson follows his duty as a soldier by killing and displacing thousands of Native Americans. Latour wants to think of him as a bad man, but he always lets him off the hook in the end because he's just a guy who's following orders.
[He] did not know just when [the New Mexico air] had became so necessary to him, but he had come back to die in exile for the sake of it. (9.3.8)
After he has lived in New Mexico for a long time, Father Latour realizes that he has a hard time being away from the place. This is where his sense of religious duty has taken him, and so it's only in this place that he feels truly fulfilled in what he has done with his life.
But the Spanish Fathers who came up to Zuñi, then went north to the Navajos, west to the Hopis […] they came into a hostile country, carrying little provisionment but their breviary and crucifix. (9.4.4)
The first priests who visited the Native American peoples of the southwest had almost nothing to live on except their sense of religious duty. Many of these men died long before their time. But Father Latour knows that they'll be remembered for the sacrifices they made.
And Father Vaillant had not been content to be a mere missionary priest. He became a promoter. (9.5.12)
Father Vaillant isn't content to give mass and be a normal priest. He is devoted to converting as many people to Catholicism as he possibly can. He considers it his sworn duty to spread the church to all corners of the world, especially the corners where it hasn't had a lot of reach in the past.