Canonisation is the process in which a Christian preacher is included in the list, or canon, of saints of the Christian Church. In case of the Catholic Church, only the Holy See has the power to declare a person a saint.
Initially, people who were martyrs — those killed in the name of Jesus Christ — were declared saints. However, the process has changed over time to include four specific steps — each with its own honorific for the person being considered — through which an individual has to pass in order to be called a saint.
Mother Teresa, who will be canonised on Sunday, Sept. 4, has passed through three of these stages. With the formalities of the final stage being completed, she will be finally called a saint on Sunday. Here is the full process she has undergone to be declared a saint:
Servant of God: The process begins at the place where the individual to be considered for possible sainthood died or is buried, when the local bishop orders that person be investigated on the value of their virtues. The norm is to begin this stage five years after the person's death, but the Pope has the powers to get the ball rolling even later — as was the case with mother Teresa.
In this stage, the individual, now called a "Servant of God," is thoroughly investigated through their writings, people's eyewitness accounts and other forms. At some stage, their body is also exhumed to ascertain whether they had been subjected to any ritual that would negate the entire process.
Venerable (Also: Heroic): Sufficient information needs to be gathered about the Servant of God person to prove their theological virtues — faith, hope and charity — and cardinal virtues —prudence, justice, fortitude and temperance. Such people can have prayer cards made of them, but the churches in their name come at a later stage of the process.
Blessed: This is the stage Mother Teresa is at. Any person who had been chosen as a Venerable and declared by the Pope to have been martyred or died for Christianity would automatically qualify for this title. For others, it would take literally a miracle to attain this position. It would have to be proven that the God had performed a miracle on prayers sent to the Venerable.
These days, the miracles are mostly of medical in nature, with people being cured of diseases or ailments deemed incurable by doctors. Once this miracle is established, a Venerable becomes a Blessed.
Saint: The Catholic Church declares a person as saint when it believes that the individual communicates directly with God — a phenomenon called Beatific Vision. This requires establishment that the Blessed has performed at least two miracles, although that may not be always necessary, as was the case with Saint Joseph Vaz.
Mother Teresa's second miracle was recognised by the Pope in December 2015, as a result of which she is being canonised.
Bonus fact: The process of canonisation used to require a person to argue why a Blessed should not become a saint, based on the merits of their life and actions. This person was called the Devil's Advocate, from where the common English idiom originates. The position was abolished as part of a series of reforms in the 1980s.