ROME — For many, it’s an odd idea that the Catholic Church has at its core a sovereign state that enjoys diplomatic relations with a robust total of 180 countries. Yet the Vatican’s diplomatic corps is among the oldest in existence, with some of its bilateral relationships, such as with Spain, dating to the 15th century.

More than once, people have questioned the need for the Vatican to have such a sovereign standing.

Yet arguments in favor of the Vatican’s unique status are also abundant, including its track record for making a difference: Both the United States and Cuba credited Pope Francis and the Vatican’s ultra-discreet diplomacy, for instance, with helping broker a December 2014 deal to restore relations.

Thus, when Pope Francis gave his annual address to diplomats accredited to the Vatican on Monday, he did so as the head of an entity whose sovereign status isn’t just a quaint anachronism, but a political tool that packs a real punch.

Although the Holy See is closely associated with Vatican City, the two are separate entities. The Vatican City State is the independent territory over which the Holy See is sovereign, and it came to be only after the 1929 Lateran Treaty that ended a breach between the papacy and the newly unified Italian state.

It’s the Holy See, not Vatican City, that has diplomatic relations with states and with 16 inter-governmental bodies, including the United Nations, the African Union, and the Organization of American States, and it’s the Holy See to which foreign envoys are accredited.

According to the Vatican’s yearbook, states have been sending diplomatic representatives to the Holy See since the late 15th century. The Vatican, however, has been sending envoys to other countries since at least the 5th century.

Bilateral relations have been growing steadily from the 19th century, but the real explosion came under St. John Paul II. During his almost three-decade-long papacy, the number of nations with full ties to the Holy See went from 85 to 174. Among those who signed up were the United States (1984), Great Britain (1982), and Mexico (1992).

Of the 180 countries that have diplomatic relations with the Holy See, only 80 have embassies in Rome.

Since the same ambassador can’t be accredited both to the Holy See and to Italy, the 78 states that don’t have a separate mission to the Holy See, such as Uganda and Nepal, have Vatican envoys based in other European cities. Other countries have diplomatic relations with the Holy See but no embassies, such as South Sudan and Turkmenistan.

The ban on one person doubling as ambassador to Italy and the Holy See dates to the Lateran Treaty, and it’s designed to guarantee the independence and sovereignty of the Holy See.

The fact that Rome contains the capitals of two sovereign states causes some diplomatic oddities: Italy doesn’t recognize the State of Palestine or the Republic of China (known as Taiwan), for example, yet both of those governments have embassies to the Holy See in the city.

For diplomats, the main point of contact is the Secretariat of State, headed by Italian Cardinal Pietro Parolin.

The secretariat is divided into two parts: the Section for General Affairs, which deals with Church affairs, and the Section for Relations with States for conventional diplomacy. Its current head, in effect the Vatican’s foreign minister, is British Archbishop Paul Gallagher, a former papal representative in Burundi, Guatemala, and Australia.

When it comes to the United Nations, the Holy See is a “permanent observer” rather than a voting member. However, it participates in many UN conventions, such as those on racial equality and the rights of the child.

Bilateral relations also mean that the Holy See maintains 180 diplomatic missions abroad, of which 106 are residential. In these countries there’s a papal ambassador, known as a “nuncio” from a Latin word meaning “message,” who often is accredited not only to that country, but also to a handful of other nations or international organizations.

The papal embassies are known as “apostolic nunciatures.” During trips abroad, popes usually stay in these nunciatures.

Nuncios, most of them archbishops, are often the deans of the diplomatic corps in the countries in which they’re stationed, an arrangement that dates to the 1815 Congress of Vienna, ratified in the 1960s. When it comes to the diplomats accredited to the Holy See, the dean is the envoy who has served the longest.

For the Vatican, there are a handful of nations with which resuming, or establishing, diplomatic relations is a keen priority, with arguably no country ahead of the People’s Republic of China (mainland China).

The Holy See and the Chinese government have been engaged in a decades-old diplomatic impasse, related mostly to the selection of new bishops in China’s state-sanctioned Catholic churches. The dicey nature of things was evident in late 2014, when Francis refused to meet the Dalai Lama. A Vatican spokesman said at the time the request was declined “for obvious reasons concerning the delicate situation” with China.

Other countries without diplomatic ties to the Vatican are Saudi Arabia, North Korea, Vietnam, Afghanistan, and Bhutan.