Those qualifications are generally fairly minimal — reasonable intelligence, awareness of the customs and practices of the host nation and the overall policy of the United States towards that nation and its region, ability to adhere to protocol and local norms of polite behavior among elites, and exercising a moderate amount of administrative oversight of professional diplomatic staff. Ms. Kennedy appears to possess all of these attributes and is Catholic to boot.
So on the one hand, I’m kind of taken aback that the Vatican is refusing to recognize her appointment. It is not normally a requirement that the two nations agree on everything — in fact, I can think of no nation on Earth with whom the United States has 100% agreement on all issues. The ambassador’s challenge is not to compromise our policies away or even to particularly search for a way to make the other nation change its policies. Rather, it is to explain away and smooth over the policy differences so that the nations can work together smoothly towards their common goals and participate together in the international community. I can see no reason why Ms. Kennedy could not do this with the Vatican.
But then, I get more taken aback by the question of why the Vatican gets its own ambassador at all. The Roman Catholic Church is the only religion on Earth that is also its own nation. A small one, to be sure, but at the same time one created by way of a treaty between Benito Mussolini and a Nazi collaborator. (To be fair, that agreement has since been substantially amended. But the point remains.) There is no ambassador to Judaism, Lutheranism, Buddhism, or any other religion one might think of. Certainly there are diplomatic relations to other nations that have official state religions (e.g., Saudi Arabia, Singapore, Egypt); there are also relations with nations that have secular governments but large religious populations overtly exerting political influence (e.g., Turkey, Israel, India).
But if we had no ambassador to the Vatican, relations with that less-than-one-square-mile patch of land — even jam-packed as it is with administrative offices for the world’s largest organized religious sect, spectacular world-class artwork in some cases built in to the architecture, the world’s finest library of ancient and medieval texts, archeological sites of tremendous significance, and a very nice set of apartments for a wealthy, older, unmarried German gentleman and his household staff — would obviously be handled under the direction of the ambassador to Italy.
For instance, if some American tourist misbehaved while in the Vatican, that’s a problem for the Holy See since there is, to my knowledge, no jail or court in the entire complex. So the misbehaving American would be arrested by the Pope’s Swiss Guards and then handed over to the local detachment of carabanieri, and dealt with, or not, according to the Italian justice system. So that’s something that the Italian authorities would deal with. You don’t need a passport or a visa to visit the Vatican and in fact there is no immediately clear demarcation of where Italian territory ends and where the Holy See’s territory begins. You just walk down the street and then wham! you’re in the piazza and all of a sudden the police are wearing what seem like outlandish costumes.* But, like I mentioned before, all they’d do is hand you over to the Italians if you got out of line.
Obviously, it’s a political sop to Catholics in the U.S. that we do it at all. And no other religion, not even any other sect of Christianity, gets that kind of treatment. Doesn’t seem fair to me — almost like the government is playing favorites.