Charles was born in 1538 near Milan. He was born wealthy — in fact, he was part of the famously rich and influential Medici family — but sought to use his wealth to benefit the Church rather than himself.
Owing in part to his well-connected family, Charles soon assumed staggering responsibilities, serving as a papal diplomat and supervisor of major religious orders.
Charles was a central figure in the Council of Trent (1545–1563), which among other things served as the Church’s official answer to the Protestant Reformation. Its twofold mission was to clarify Catholic doctrine against Protestant objections and reform the Church internally against many long-standing problems. As a papal representative, Charles participated in the council's conclusion in 1563, when he was only 25, and was ordained a priest during the council. He also played a leading role in assembling its comprehensive summary, the Catechism of the Council of Trent.
Charles’ uncle, Pope Pius IV, appointed him archbishop of Milan in 1563, and soon after he became a cardinal. He found his diocese in a state of disintegration, after two generations of virtually no local administration or leadership. Charles got straight to work establishing schools, seminaries, and centers for religious life. He constantly directed the work of restoration of ecclesiastical discipline, and the education of the young, even down to minute details. He tried as much as possible to live a simple life and give to the poor whenever possible, and he practiced self-mortification.
The clergy during this time were in many cases lax and careless, living scandalous lives, such that the people had grown to be equally negligent and sinful. While bishop of Milan, St. Charles oversaw many dramatic and effective reforms of the clergy, the liturgy, and of religious education. He encountered much opposition to those reforms, so much so that a group of disgruntled monks attempted to kill him, but he was miraculously unharmed when an assassin fired a gun straight at him while kneeling in prayer at an altar.
He was very active in preaching and ministry and was famous for bringing back many lapsed Catholics to the Church. As a result, today he is honored as the patron saint of catechists and catechumens, people who teach and learn the faith. In fact, he was the founder of the Confraternity of Christian Doctrine, which systematically instructed children in the faith — the forerunner of the modern “Sunday school.”