From Websites Dedicated To Her:
Thérèse became a saint because of the way she responded wholeheartedly to God’s love and the grace of the Holy Spirit within her. We believe that God embraced her upon her death and shared with her a risen life of Jesus in heaven. While the Church recognizes the holiness and sanctity of many good people, official canonization is a long process. The Roman Catholic Church seeks definite signs before she officially declares someone a saint. In the case of St. Thérèse, the process went swiftly. In the popular piety of the people she became a saint in their hearts even before the official declaration.
A person is declared “Venerable” when there is evidence of extraordinary holiness and inspiration for others. It means that the cause of their canonization is being pursued. In order to move to the next step “Beatification”, two certified miracles are needed, attributed to the intercession of that person with God in heaven. Miracles must involve situations where there is no other natural explanation. They are evidence of supernatural intervention, through intercessionary help.
Regarding St. Thérèse, in 1923 the Church approved of two spontaneous cures unexplained by medical treatment. Sister Louise of St. Germain was cured of the stomach ulcers she had between 1913 and 1916. The second cure involved Charles Anne, a 23 year old seminarian who was dying from advanced pulmonary tuberculosis. The night he thought he was dying, Charles prayed to Thérèse. Afterward, the examining doctor testified, “The destroyed and ravaged lungs had been replaced by new lungs, carrying out their normal functions and about to revive the entire organism. A slight emaciation persists, which will disappear within a few days under a regularly assimilated diet.” These two miracles resulted in Thérèse becoming beatified.
Once she was declared “Blessed”, it took only two years for the necessary next two miracles to be approved. In 1925 two cures had been investigated and judged to be supernatural, through the intercession of St. Thérèse. The first involved Gabrielle Trimusi from Parma, Italy. Gabrielle had suffered from arthritis of the knee and tubercular lesions on the vertebrae. The final cure involved Maria Pellemans of Schaerbeck, Belgium.
Maria suffered from pulmonary tuberculosis which had spread, as Thérèse’s illness had, to the intestines. The diagnosis of pulmonary and intestinal tuberculosis was made by a Dr. Vandensteene, who also examined Maria after she came back from visiting Thérèse’s grave. The doctor testified, “I found Miss Pellemans literally transformed. This young woman, out of breath from the least movement, moves about without fatigue; she eats everything given to her, with a very good appetite. The abdomen presents no tender point, when formerly the least pressure produced severe pain. All symptoms of tubercular ulceration of the intestine have disappeared.” In reports predating Maria’s return to health, two other physicians confirmed Dr. Vandensteen’s diagnosis of pulmonary and intestinal tuberculosis. On May 17, 1925, Thérèse was officially declared a Saint by Pope Pius XI.