One of the most highly regarded saints of the Catholic Church is St. Thomas Aquinas, often referred to as the "Angelic Doctor."
Why is he called a "doctor"?
First of all, in the Catholic Church, besides naming certain individuals as saints, recognizing their extraordinary practice of virtue, popes over the centuries have singled out specific saints as Doctors of the Church.
These men and women are seen as exemplary teachers in various theological or spiritual topics. The official title of doctor stems from the Latin root word docere, meaning “to teach.”
Aquinas wrote extensively during his lifetime, producing one of the most foundational texts for Catholic theology, called the Summa Theologiae. In it, he answers a variety of questions that were raised in his time, and his responses continue to be used today in defense of Catholic doctrine.
What about "angelic"?
It's not entirely certain why St. Thomas became known as the "Angelic Doctor." There are a variety of possibilities, including one explanation given by Pope Benedict XVI in a general audience in 2010.
The Catholic Encyclopedia offers a similar explanation.
This entry refers to a story from St. Thomas' life, recounted in his canonization records.
St. Thomas also wrote extensively about angels, and his theology on angels is one of the most comprehensive in all of Church history.
For all these reasons, St. Thomas Aquinas is known as the "Angelic Doctor."
