VIDEO: What makes someone a Doctor of the Church?

Why is St. Francis de Sales honored as a Doctor of the Church, but not St. Francis of Assisi? Why are St. Teresa of Avila and St. Therese of Lisieux Doctors of the Church, but not St. Teresa of Calcutta?

In 2,000 years of Church history only 36 saints have been designated as Doctors of the Church. And there are certain criteria that set these saints apart. Find out what they are: