I've been continuing my research on this and am always adding sources to my current collection to refine both my knowledge and arguments, so I'd like to share some recent findings. The citation in question here is
Missionaries of Republicanism: A Religious History of the Mexican-American War by John C. Pinheir:
The Philadelphia Public Ledger, for example, argued that the Spanish Catholic nature of Mexico would create problems in the future if the country were not annexed quickly. This refuted the prevailing view that the white Anglo-Saxon United States could never absorb a non-white race of people as social and political equals. John L. O’Sullivan, who had introduced Americans to the term “Manifest Destiny” in 1845, now argued that because the “virtues of the Anglo-Saxon race make their political union with the degraded Mexican-Spanish impossible,” the only choice was to “amalgamate” the races through the work of “missionaries of republicanism.”3
Further on:
Whig opinion of the war vacillated to some extent but evolved relatively little prior to the onset of Scott’s Mexico City campaign. In their opposition to what was already being called “Mr. Polk’s War” by its opponents, they resorted to arguments and vocabulary rooted in racialist, anti-Catholic beliefs. Whigs generally opposed westward expansion for constitutional and practical reasons, believing that too diverse and far-flung a country posed a danger to American identity. They also had legitimate worries that adding new states in the West would diminish their political power. During the war, however, Whigs emphasized none of these concerns. Instead, they tended to stress religion and race in their arguments against annexing Mexican territory. Whigs doubted whether “eight millions of men at war with us by race, by language, by religion, manners, and laws” could ever become virtuous American republicans.
Aware that increasing numbers of evangelical Protestants were supporting the war at least as a means of evangelizing Mexicans, Whig leaders cognizant of evangelical influence in their party acknowledged the war’s one potential benefit: rescuing “seven millions of people from extinction by sowing among them the seeds of a true Christian faith.” Even so, saving Mexican souls was not the Whig leadership’s primary concern. The largest pitfall of any Mexican conquest, they believed, grew out of the old belief enunciated by John Jay in Federalist #2 that republics needed to be homogenous in culture, language, religion, and custom in order to prosper and not disintegrate. Thus, Whigs simply denied that a republican endeavor comprised of two different peoples—Americans and Mexicans—could succeed: “even if annexation were good for Mexico, it would [be] bad for us.”6
Further:
The Rev. John N. Maffit occupied the extreme pro-war position. A Methodist Episcopal minister, Maffit hailed from New York but had lived in Tennessee and Kentucky. In 1841 he had served as chaplain to the U.S. House of Representatives. By that time he had already become famous (or, rather, infamous) for his controversial preaching on the Southern lecture circuit. In the summer and fall of 1847, the Rev. Maffit embarked on a speaking tour of a number of cities, including New York, Cincinnati, and Philadelphia. He preached primarily about the war and argued “that the conquest...is part of the design of Providence for reforming the religion and morals of that country.” Indeed, Maffit’s anti-Catholicism ran so deep that he unapologetically supported not only the war, but also the conquest of all Mexico strictly for the purposes of evangelization.39
Preachers like Maffit even made their way to Mexico as volunteer chaplains. One of these, Richard A. Stewart, was a sugar planter and Methodist minister from Louisiana. Stewart was so strongly in favor of the war for religious and expansionist reasons that he captained his own volunteer company. His fervor in Mexico matched Maffit’s in the United States. Stewart preached a sermon on the eve of the conquest of Matamoras that quickly became famous. In it, he portrayed America as Israel and 144 Missionaries of Republicanism “the land of Mexico. .. as the land of the Canaanites” waiting to be conquered.40 Evangelical pacifists pointed to Stewart’s Matamoras sermon as the inevitable outgrowth of the proposition that the war could be turned toward the good end of opening Catholic Mexico to the Gospel. Stewart’s rationale, however, was little different than that which underlay most Anglo-Saxonist, Manifest Destiny rhetoric.41
Further:
Most Protestant preachers who were also anti-Catholic occupied a middle ground between the uncompromising anti-war stance of Parker and the fiery pro-war rhetoric of Maffit. They censured the war as unnecessary and evil but acknowledged that “God was going to bless the next generation of Mexicans through the wickedness of the American nation.” Ministers from every geographical section of the country voiced such sentiments. Even those who did not believe in Manifest Destiny or trumpet the Anglo-Saxon race assured the faithful that the war would “result in good to Mexico” because it “may be the way by which God means to disenthrone the man of sin” from North America. Others, in explaining how “evil shall be overruled for good,” pointed out that the war would “push forward also the principles of civil and religious liberty.”43