If you are curious look up the Protestant Reformation, Martin Luther and his 99 Theses.

I'm paraphrasing and my thoughts on what I experienced this but it came down more to the idea that Catholics worship the pope and the saints more than god and jesus. If you were the leader of a nation that called themselves Catholic you could find the Pope telling you what to do when it came to war and if you declared war on another Catholic country the Pope could tell you to stop or to declare peace. To not do so was in danger of having all other Catholic nations declare war.

Not to mention the many saints you are required to pray to, Purgatory and praying for the dead, all of the rituals, services in Latin, worship of the virgin mother, the schism that split the church between two Popes who excommunicated each other, etc.

Protestantism did away with all of that. No single leader, the ability to create different sects that didn't make you an apostate of the church, etc. Now don't get me wrong even the same sect don't always believe the exact same things and it can get pretty nit-picky, but Protestantisn can change with the times more easily than Catholicism can.

The goal was to make less of a ritual cult like Catholicism had become, and more of a focus on the meaning of the the actions of jesus, being able to actually get to heaven without all of the pomp and circumstance that really meant nothing, and all that crap.

The worry is the President would be more loyal to the other Catholics than the rest of the nation and would be bound by cult rules than the will of the people.

Ironically enough right now it's the Catholic President trying to stop rights from being taken away while there are both Protestants and Catholics in the Supreme Court and other facets of the government that are working so hard to do the opposite.