How hard was it for Italians, who were mostly Catholic, to agree to take the papal state, was there a large resistance within Italy? I know that many States in Europe were not Catholic at the time and that the governments were more secular, but surely there must have been a sizable part of the population of Europe and abroad (like the Americas) that was upset at the Italians.
I can't speak for the whole planet, but Italians (and many others) were decidedly mixed on the subject. The 1860's conflict with the Papal States were just one of a long succession of armed conflicts between Catholics and the Pope. So in a lot of ways it didn't look that different than previous events. For instance, Pius IX was the reigning pontiff during this period. In 1848 he was forced from Rome and was eventually restored. So the papacy being surrounded in 1861 and losing Rome a decade later didn't immediately declare that this was something very different.
The reality was different, though. From 1870 to 1929 this new situation was called "The Roman Question." For its part, the new Italian state tried to regulate proceedings with the unilateral declaration of the "Law of Guarantees." This document was categorically ignored by the papacy. Its guarantees of papal independence, its requirements for bishops to be approved by the new Italian king, and its offers of a yearly stipend were all rejected by Pius IX and his successors. The popes considered themselves "prisoners" in the Vatican. For a long while, there was a declaration by the pope that Italian Catholics could not participate in the new Italian state. This policy was called Non Expedit, and it was unevenly received.
The papacy and many Catholics expected the same thing to happen that had always happened in such situations--eventually a European power would find it in its best interest to restore papal control over Rome and/or the new government in charge of Rome would fall apart. This (somewhat obviously) never happened. Overall, those Catholics who were particularly annoyed at the new Italian state did not sufficient motivation or support to do anything other than complain--there was no armed resistance that I am aware of (there had been some Catholics who flocked to the pope's banner prior to 1871, but I don't know of any after that date).
And why would there be? For two thousand years (very roughly) the Pope was a religious leader and the head of a temporal state. Surely things would return to the status quo ante bellum, right? Nope. This was a new situation. It took nearly sixty years for the situation to be resolved by the Lateran Accords, and even longer than that for the papacy (and by extension, Catholicism) to adjust to the new reality of the pope being the head of a tiny temporal nation instead of a regional power.
I hope that answers some of your questions. As always, followup questions by OP and others are highly encouraged.