There are 33 Doctors of the Church. To be made a Doctor, a saint is said to have had a special insight into some teaching or doctrine or a new way of understanding God. St Laurence preached against the Reformation and expounded the teachings of the Council of Trent. It was his ability to articulate the teaching of the Church which led to him being a Doctor.
Below is a writing about him I found which makes good reading:
St. Lawrence of Brindisi - July 21
Prof. Plinio Corrêa de Oliveira
Cesare de Rossi was born at Brindisi in the Kingdom of Naples on July 22, 1559 into a Venetian family of a good social level. When sixteen, he entered the Capuchins, taking the name Lawrence. After studies in Verona, he was ordained and began to preach with great effect in Northern Italy. In 1596 he went to Rome as General of his Order, a position he would hold five times.
St. Lawrence of Brindisi
At the request of Emperor Rudolf II, who knew the reputation of the Capuchin, Pope Clement VIII sent Fr. Lawrence to Germany to help raise a crusade against the Turks. At that time, Mohamed III, the sultan of Constantinople, was advancing beyond the Danube with the aim of invading Hungary and Austria, thus opening the way to enter Italy. He bragged he would transform the altar of St. Peter Basilica into a manger to feed the Turkish horses.
Emperor Rudolf had organized his army and invited all the German Princes to join him to defend Christendom. Fearing that this invitation would not be well received, he sent Fr. Lawrence to solicit assistance from the German Princes. The Capuchin was successful, and the help soon arrived. Archduke Matthias was chosen as the general of the Christian army. At his request, the Pope ordered Lawrence to join the army as its chaplain and adviser. He obeyed without hesitation.
When he arrived at the battle camp in October 1601, he found the army lined up in battle formation before him. The holy religious, a cross in his hand, addressed the soldiers and assured them a certain victory. Then, he prepared them for the combat by means of prayer and penance.
The Commander in Chief
On the day of battle, the Catholic General had only 18,000 men before the 80,000-strong army of the Turkish Sultan. Faced with this great disparity in numbers, the secondary commanders advised prudence and counseled the Archduke to withdraw. Archduke Matthias sent for Fr. Lawrence to ask his recommendation. The Capuchin advised attack, and for a second time, he assured a complete victory for the Catholic forces. His ardent faith wiped away the fear of the commanders. The decision was made to give battle, and the soldiers took their positions. Mounted, Lawrence addressed the troops and spoke with such vigor that the troops took the initiative and drove forward to attack the Turks with an incredible force.
The Turks resisted this first attack. Fr. Lawrence, who was riding in the first line of attack, continued to encourage and exhort the troops: “Advance! Advance! The victory is ours!” The soldiers charged again, and the enemies fell back in terror, fleeing in every direction. The Battle of Stuhlweissenburg took place on October 11, 1601. On the 14th of that same month, the Catholic forces engaged the Muslims in another battle and won a complete victory. The Turks withdrew their army behind the Danube after suffering a loss of around 30,000 men.
It is difficult to express the admiration Fr. Lawrence inspired among the commanders and soldiers. The Duke of Mercoeur, one of the commanders, declared that the holy religious had done more to win the war than all troops together, and that after the help of God and the Virgin Mary, it was to him that the two victories were owed. Once again Europe had been saved from the barbarian Muslims.