After taking a look at the life of St. Cyril, it’s easy to see him as a man who always came into a situation with both barrels blazing. Seriously, St. Cyril took no prisoners. When he became Patriarch of Alexandria in 412, he “assembled a mob” that booted out a certain group of people who had been persecuting Christians in the area.
Before St. Cyril became Patriarch, he had to survive a riot that ensued due to a rivalry for the Patriarchy.
After St. Cyril became Patriarch of Alexandria, he went at it with the prefect of Egypt, Orestes, who thought Cyril was a rival to his authority and was also irate that St. Cyril kicked out of Egypt that certain group of people who had been persecuting Christians. Yes, you guessed it, a serious brawl ensued as a result of the conflict between St. Cyril and Orestes. 500 (yes, five hundred) monks came swinging out of the lower deserts of Egypt (Nitria) to defend St. Cyril. Can you imagine 500 men with big beards and worn-monastic habits storming into a fight against Oreste's soldiers? One word comes to mind: Fortitude. One of the monks actually beamed Orestes with a rock during the skirmish. Orestes had the stone slinging monk tortured; the monk died as a result.
St. Cyril honored the remains of the rock lobbing monk for a time.
St. Cyril, in league with Pope Celestine I, is most known for intellectually duking it out with Nestorius, who was the Patriarch of Constantinople (present day Istanbul). At one point, the Emperor (Theodosius II) had both Nestorius and St. Cyril arrested. The emperor, however, cut St. Cyril loose after Papal Legates showed up on his doorstep saying that Pope Celestine endorsed St. Cyril’s condemnation of Nestorius.
So what was the big deal with Nestorius? Well, he promoted the heresy of Nestorianism, which says that “Mary was not the Mother of God since Christ was Divine and not human, and consequently she should not have the word theotokos (God-bearer) applied to her.”
St. Cyril was the bedrock for the third general Council of Ephesus in 431, which declared Nestorianism a Heresy. Oddly enough, a group of bishops that sided with Nestorius convened their own council after the one at Ephesus and deposed St. Cyril (this is the point where St. Cyril and Nestorius got arrested by the Emperor).
But the biggest reason why St. Cyril of Alexandria is a ‘Trooper’ is his doctrine, which has been quoted by multiple Church councils—St. Cyril has the title Doctor of the Church. Here is an excerpt from his book on the Divine Motherhood of Mary:
“In the third book of his work on the holy and consubstantial Trinity, our father Athanasius, of glorious memory, several times refers to the holy Virgin as "Mother of God." I cannot resist quoting his own words: "As I have often told you, the distinctive mark of holy Scripture is that it was written to make a twofold declaration concerning our Savior; namely, that he is and has always been God, and that for our sake in these latter days he took flesh from the Virgin Mary, Mother of God, and became man."