During the 1850s, there was much suspicion that the Roman Catholic Church was a secret body that held its own laws and would not abide by the criminal laws of the United States. But doesn't today's rampant priest sexual-molestation scandal validate the suspicion held by the members of the pre-Civil-War Know-Nothing movement?

While it is possible to make an argument that recent sexual abuse scandals within the Roman Catholic Church validate the positions of the Know Nothing Movement with regard to Catholicism, such an argument may be logically fallacious since the specific conditions of the current episode are largely dissimilar to the assertions of Know Nothings.
The essence of the Know Nothing philosophy with respect to Catholicism was that American Catholics had greater allegiance and were more loyal to the Pope than they were to the United States and that, as a result of this allegiance and loyalty, they represented a fifth column intent of subverting American domestic institutions and seizing the levers of government.
The differences between the current sexual abuse scandals and the Know Nothing assertions, therefore, are twofold:
First, there is no consensus or widely held belief that sexual abuse of children was done under the orders or directions of the Pope nor in any organized manner. If it were true, as some have alleged, that the Pope was complicit in suppressing or hiding revelations of sexual abuse once they occurred, this would represent a reactive interference in domestic institutions rather than a proactive interference as the Know Nothings feared.
Second, the sexual abuse of children in the Roman Catholic Church was done to satisfy the prurient cravings of individual members of that Church, specifically priests, and not as part of a master scheme to seize control of the United States government.
https://ohiohistorycentral.org/w/Know-Nothing_Party

https://www.bbc.com/news/world-44209971