So this is an interesting question from the angle of religious legitimacy for actions. Yes, the Philippines were very Catholic under Spanish rule, which isn’t really surprising when you look at other Spanish colonies. Spain itself has a history of strong Catholicism with how well it defended against the Reformation. Which makes us look at the second country, the United States. The United States had a small minority of Catholics in its first stages. Julie Byrne makes the claim that Catholics were a small minority until 1845, and I have seen papers putting the Catholic Church as the largest denomination starting in the 1850s. So the Catholic Church was a minority, but it did grow to become a majority. However, America had some strong anti-Catholic leanings at times (America was settled by a lot of Protestants).

The president at the time (William McKinley) was also a Protestant (Methodist). In his interview, McKinley says that he prayed to God on what to do about the Philippines since he “didn’t want the Philippines”. We should keep in mind that Rusling was also a Methodist, which may have caused him to exaggerate the religious reasons for McKinley’s actions, but the source is still used nonetheless. Religion has been used for legitimacy for actions throughout history, and this was just another example of it. The religion being used for legitimacy here was Protestant Christianity. There were some drawbacks to this as it left a bad taste in some Catholic mouths and some of them lost their enthusiasm for taking the Spanish territories. There were Catholic anti-imperialist sentiments cropping up due to what was seen as an attack on Catholic culture.

The “Christianizing” of the Philippines was to be undertaken by Protestant missionaries. While yes, the Philippines were heavily Christian before America’s arrival, it was Catholicism. Protestants and Catholics hold slightly different beliefs, so it was more a re-“Christianizing” into Protestant Christianity. The reasons, many. Some saw economic and political benefits to the Philippines, others didn’t. The campaign for “Christianizing” was backed by Protestants though, so that’s why you see it pop up.

Sources:

“Roman and Catholic and American: The Transformation of Catholicism in the United States” José Casanova 1992

“"The Northwestern Chronicle" and the Spanish-American War: American Catholic Attitudes Regarding the "Splendid Little War"” Scott Wright 2005

“Interview with President William McKinley” General James Rusling 1903