Saint Patrick was born Patricius in Roman occupied Great Britain. He was kidnapped and sold into slavery in Ireland as a teen and worked for a local warlord as a shepherd. He escaped 6 years later and returned to Great Britain to study for the priesthood.
Patrick received a vision to return to Ireland to evangelize the land of his captivity. He did this without hesitation, in spite of the fact that no one had taken the Gospel outside of the Roman Empire.
In the five centuries before Saint Patrick, the followers of Christ had not yet taken the Gospel to "all nations", in part, because they didn't believe that the "barbarians" living outside of Roman Civilization, were fully human. Saint Patrick was the first to do this.
Patrick's kidnapping was a blessed turning point in his life. Had he not been kidnapped at that crucial time of adolescence, he would not have grown to become the man he became. As a result of his captivity, he spoke out against the enslavement of human beings centuries before the Church officially condemned it. He also saw women as equal human beings in Christ. Patrick speaks in his Confessions of
...a blessed woman, Irish by birth, noble, extraordinarily beautiful—a true adult—whom I baptized.
Had he not answered God's call to missionary work, Ireland would have remained without God's Word for an untold time in the future.
Saint Patrick was the first recorded human being to speak out against slavery and the first true Christian Missionary. By the time of Patrick's death, or soon after, Ireland stopped the slave trade and never again participated in human sacrifice. Because of Saint Patrick's work, Irish monks went on to help preserve Western Civilization during the Middle Ages.
Thanks to the Saint Anthony Messenger for much of this information.