If you've read the writing of Aquinas, angelic may not be the first thing that comes to mind. Frustrating? Yes. Deep? Yes. Angelic? Um....maybe not. But we'll get to why he's called that. 

As you also may have guessed, St. Thomas joins St. Catherine of Siena as one of the Dominicans who have been named Doctors of the Church. You shouldn't be surprised by that. Anyone who wrote as much as Thomas wrote and inspires an entire study of theology (called "Thomism", or "Thomists", for those who subscribe to his views) should definitely be ranked s one of the Church's greatest teachers. 

But in his time, St. Thomas was kind of an odd duck. Called "the dumb ox" by his brethren, because he barely spoke, Thomas was born on January 28, 1255, in Italy. Hi parents, Lundolf and Theodora, wanted Thomas to follow his uncle into the abbacy of a local Benedictine monastery. But Thomas had other ideas. 

At the age of 19, he resolved to join the newish Dominican order. His parents, displeased with this idea, tried to dissuade him, finally locking him up in his room for a year in order to prevent him being given the habit. He used this year to tutor his sisters and communicate with members of the order. Finally, his brothers smuggled a prostitute into his room, hoping she would tempt him; Thomas drove her out of the room with a flaming poker (something I always thought was a bit hard on the poor woman). That night, two angels appeared to him as he slept and gave him the grace to always remain celibate. 

Finally, in 1244, his mother relented and arranged for Thomas to "escape" his room via the window one night. Thomas escaped, and finally joined the order. The order sent him to the University of Paris, where he was a student of another great Dominican, St. Albert the Great. While there, he picked up the "dumb ox" nickname from his fellow students. Since he didn't talk much, his fellows thought he was stupid. Upon hearing this, St. Albert said,

"You call him the dumb ox, but in his teaching he will one day produce such a bellowing that it will be heard throughout the world." 

Thomas taught in Cologne as an apprentice professor, teaching students about the books of the Old Testament, and writing commentaries on the books of  Isaiah, Lamentations, and Jeremiah. He received his master's degree from the University of Paris and continued to write various theological works, including Summa Contra Gentiles, one of his most famous works. 

In February 1265, Thomas was summoned to Rome to serve as the papal theologian (a post first held by St. Dominic, and held by Dominicans ever since) for the newly-elected pope, Clement IV. Thomas also taught the students natural and theological science at Santa Sabina. He traveled between Paris and Rome many times over the years, filling various posts and teaching. 

In 1272, Thomas retired from teaching at the University of Paris and returned to Naples, where he lived the rest of his life. Here, he worked more on his greatest work, the Summa Theologica, and gave lectures. One night during prayer, he had a vision of Christ. "You have written well of me, Thomas," Christ said. "What reward would you have for your labor?" Thomas answered, "nothing but you, Lord." After this, Thomas had a vision of some sort, but he never told anyone what it was--only that after the vision, everything he had written suddenly seemed "like straw" to him, and he abandoned the Summa, never finishing it. He died on March 7, 1274, while giving commentary on the Song of Songs. Thomas was canonized 50 years after his death by Pope John XXII. 

The Summa, while unfinished, is one of the greatest theological works of all time, and one of the classics of western literature. It was intended as a guide for theology students (that's right, beginners!), and was a compendium of all the teachings of the Catholic Church. It includes topics such as the existence of God, creation, man, man's purpose, Christ, and the sacraments. It's broken into three major parts: 

1. The first part: Prima Pars: God's existence and nature; the creation of the world; angels; the nature of man
2. The second part: Secunda Pars: broken into two subparts:
   1. Prima Secunda: general principles of morality and a theory of law
   2. Secunda Secundae: morality in particular, especially virtues and vices
3. The third part: Tertia Pars: the work and person of Christ; the sacraments; the end of the world (unfinished)

His feast day is January 28, and he is the patron of academics, apologists, protection against storms, book sellers, Catholic schools, chastity, learning, pencil makers, philosophers, publishers, students, and theologians. He's called the angelic doctor because of his angelic purity, his writings on the angels, his angelic wisdom, and angelic piety.