St. Ambrose and Theodosius.
Background
Ambrose was a Roman consular prefect in Italy. Basically a Roman governor. After the death of the old Bishop of Milan he went to oversee the election for his replacement so that Nicene Christians and Arians didn't riot and murder each other as the Arian controversy was a serious issue in the area. While there the crowd spontaneously nominated Ambrose, an unbaptized catechumen to be the new Bishop of Milan. So within a week he went from unbaptized Roman governor to Bishop of Milan, the current capital of the Western Roman Empire. Just as he took his duties as consular prefect extremely seriously, he took his new duties as bishop seriously. As such he became a staunch enforcer of Nicene orthodoxy.
Theodosius could probably be considered the first truly devout Christian emperor and would be the last emperor of a unified Rome. Subsequent to his reign it would always be split into the Western and Eastern empires. After the story below he would go on to defeat a sizable usurper army led by pagan sympathizer/revivalist Eugenius. Eugenius and his army affixed pagan deity imagery to their shields and banners. Theodosius was on the brink of defeat and the future of Christianity uncertain when a divine wind blew against Eugenius and his army. Theodosius won. The last part is disputed by secular historians, but given that it makes the story of the Battle of the Frigidus infinitely cooler, I choose to believe it.
Story
In the city of Thessalonica a famous charioteer was arrested for homosexual rape. The people of Thessalonica, being more concerned with awesome chariot races than piddly small crimes like rape, demanded he be let go. The Roman officials refused, so the citizens started to riot and multiple people were killed. When Theodosius heard about this he was furious. He flew into a rage and sent a letter ordering an army to the area to ruthlessly suppress the riots. He very quickly regretted his harsh reaction and tried to recall the order. Sadly the new orders were not received. The army slaughtered several thousand citizens of Thessalonica without any concern for guilt or innocence.
Some time later Theodosius tried to go to mass in his capital city of Milan. But the bishop Ambrose refused to allow him to enter. Let me repeat that, a random bishop refused to allow the emperor of the most powerful political entity in the world to attend mass in the city which was the seat of his power. Ambrose explained to Theodosius that the massacre he ordered was a horrific crime and that his soul was in serious peril. He explained that unless Theodosius honestly repented and paid serious penance he would surely burn.
Theodosius could have ordered the doors battered open and Ambrose executed. But he didn't. He withdrew to his palace and spent months sobbing, knowing he was doomed. Eventually he worked up the courage to speak again with Ambrose. Let me repeat that, eventually the most powerful man in the world worked up the courage to speak again with a random bishop. Ambrose agreed to lift what was essentially an excommunication on the condition that Theodosius spent months in penance and agreed to a law providing for a delay in executions so that any future knee jerk orders could be called back before any bloodshed.
Conclusion
Ambrose's background and this story are why he's probably my favorite saint. He was a man with an incredible force of will, unshakable bravery, and a strong sense of duty. As consular prefect his duty was to keep the peace, so he kept peace between the Arian heretics and Nicene Christians. As bishop his duty was to abide by and enforce the will of God, so that's what he did. The circumstances didn't matter, the odds against him didn't matter. He had a duty and he did it.
You can actually read the letter he sent to Theodosius after the massacre at Thessalonica. Ambrose is also credited with converting Augustine to Christianity and educating him, so his influence is pretty huge.