While several saints are called "Doctors of the Church," St. Bonaventure has been signaled out as the "Seraphic Doctor."
Why is that?
In Pope Benedict XVI's general audience on March 17, 2010, he gave several reasons why St. Bonaventure is known as the "Seraphic Doctor."
First of all, he explained the origins of the word "seraphic."
St. Bonaventure interprets [the] orders of angels as steps on the human creature's way to God. Thus they can represent the human journey, the ascent towards communion with God. For St Bonaventure there is no doubt: St Francis of Assisi belonged to the Seraphic Order, to the supreme Order, to the choir of seraphim, namely, he was a pure flame of love.
In many of St. Bonaventure's writings, he emphasized love of God, and wrote a prayer after communion that highlights that concept.
Secondly, St. Bonaventure is known as a "doctor" for his remarkable intellect, which Benedict XVI argued was on par with that of St. Thomas Aquinas.
He is an eminent theologian who deserves to be set beside another great thinker, a contemporary of his, St. Thomas Aquinas. Both scrutinized the mysteries of Revelation, making the most of the resources of human reason, in the fruitful dialogue between faith and reason that characterized the Christian Middle Ages, making it a time of great intellectual vigour, as well as of faith and ecclesial renewal, which is often not sufficiently emphasized. Other similarities link them: Both Bonaventure, a Franciscan, and Thomas, a Dominican, belonged to the Mendicant Orders which, with their spiritual freshness, as I mentioned in previous Catecheses, renewed the whole Church in the 13th century and attracted many followers.
St. Bonaventure's example and writings rightly earned him the title of "Seraphic Doctor."