A person becomes an official saint when she or he is canonised. The process of canonisation was established over 800 years ago by Pope Gregory IX as a way of ensuring that the person was truly worthy of being called a saint. The Congregation for the Causes of the Saints oversees the entire process.
When a person dies who has "fame of sanctity" or "fame of martyrdom," the Bishop of the Diocese usually initiates the investigation. One element is whether any special favour or miracle has been granted through this candidate saint's intercession.
Throughout this investigation the "general promoter of the faith," or devil's advocate, raises objections and doubts which must be resolved.
If the candidate was a martyr, the Congregation decides whether she or he died in a sacrifice of love for Christ and the Church.
In other cases, the Congregation decides if the candidate was an exemplary, charitable and heroic individual. The candidate's writings are also investigated to see if they possess "purity of doctrine," essentially, nothing heretical or against the faith. All of this information is gathered, and submitted to the Congregation for their deliberation.
Once a candidate is declared to have lived life with heroic virtue, he may be declared Venerable.
The next step is beatification. A martyr may be beatified and declared "Blessed" by virtue of martyrdom itself. Otherwise, the candidate must be credited with a miracle. In verifying the miracle, the Church looks at whether God truly performed a miracle in response to the intercession of the candidate saint. Once beatified, the candidate saint may be venerated but with restriction to a city, diocese, region, or religious family. Accordingly, the Pope would authorise a special prayer, Mass, or proper Divine Office honouring the Blessed.
After beatification, another miracle is needed for canonisation and the formal declaration of sainthood.
God shows to men, in a vivid way, His presence and His face in the lives of those companions of ours in the human condition who are more perfectly transformed in the image of Christ. He speaks to us in them and offers us a sign of this kingdom to which we are powerfully attracted, so great a cloud of witnesses is there given and such a witness to the truth of the Gospel. It is not merely by the title of example that we cherish the memory of those in heaven; we seek rather that by this devotion to the exercise of fraternal charity the union of the whole Church in the Spirit may be strengthened (Vatican II ’Lumen Gentium’ No. 50).
Adapted from