Ambrose was born of a Roman family, and his father was high up in Roman administration.
He himself became Prefect of the Roman regions of Liguria and Emilia; whose capital was Milan.
At this time Ambrose was not yet Baptised. He was a catechumen, i.e. a person preparing for Baptism.
The Bishop died. There was conflict between Catholics and Arians at the time. Now Arians were heretics who denied the Divinity of Christ.
Ambrose was trying to pacify this conflict. The people demanded that be became the Bishop. After pressure from the people, as well as the Emperor, he agreed. He was Baptised, Ordained, and installed Bishop within a week, on the 7 December, 374.
He immediately gave his money to the poor and started studying theology.
He was a very dedicated pastor, acting with love and integrity in his dealings with all.
He even stood up to the Emperor when he needed to. He attacked the Arian heresy with learning, firmness, and gentleness.
He was a key figure in the conversion of the great St. Augustine.
Here are some quotes from St. Ambrose:
There is nothing evil save that which prevents the mind and shackles the conscience.
In some cases, silence is dangerous.
When you are at Rome live in the Roman style; when you are elsewhere live as they live elsewhere.
When I am in Rome, I fast on Saturdays (local custom). When I am in Milan I do not. Do the same. Follow the custom of the Church where you are.
Prayer is the wing wherewith the soul flies to heaven, and mediation the eye wherewith we see God.
He alone can take all sorrow away from my heart.
No-one is good but God alone. What is good is therefore divine, what is divine is therefore good.
God created the universe in such a manner that all in common might derive their good from it, and that the earth should also be a property common to all.
We learn many things from St. Ambrose. Among these things is the reminder to continue learning about our faith. We also learn to be integral and not to let what others think stop us from doing what is right.
Also, our example can greatly influence others, such as St. Augustine was influenced by St. Ambrose.