Typically when the Vatican declares an individual a saint, there is a standard canonization procedure that must be followed.
The individual's life needs to be examined thoroughly to determine if they lived a life of heroic virtue, and evidence of two verifiable miracles that further confirm the saints' heavenly intercession must be documented.
This process normally takes many years to complete, and the miracles need to go through an in-depth scientific inquiry.
However, if that saint has been deceased for several hundred years, the procedure is more difficult to follow, so the Vatican has the option to declare someone a saint through "equipollent canonization."
The “equipollent” canonization procedure
As explained in a previous Aleteia article, with the “equipollent” canonization (sometimes called "equivalent" canonization), the inscription of the saints is done by a simple decree of the pope without the regular process, which involves verification of a miracle. These saints “must have enjoyed a long and uninterrupted devotion, as well as a reputation for signs and graces.” Their canonization is requested by a large representation of the Church, directly to the pope.
Benedict XVI used the same recognition for St. Hildegard of Bingen, who is also recognized as a doctor of the Church.
Benedict XIV formalized this process in the 1700s. Through it, a pope invites the universal Church (as opposed to the local place where a saint might have been from, or a particular religious order he or she belonged to) to recognize the feast of the saint, with Mass and the Divine Office.
This recognizes that the Church’s judgment has been made on the person’s sanctity, even though the usual formula of canonization hasn’t been followed.
An equipollent canonization is still very rare and only occurs with a very small percentage of saints.