Calvert entered politics in the early 1620s and was elected to Parliament in 1624 and 1625. He was a supporter of the Protestant Reformation and was opposed to the Roman Catholic recusancy policies of King Charles I. In 1625, Calvert was arrested and imprisoned for his religious beliefs, but he was later released and made a member of the Privy Council.
In 1629, Calvert was granted a royal charter for the Province of Maryland, which he named in honor of his wife's mother, Henrietta Maria of France. The charter gave Calvert and his heirs the right to govern Maryland and to establish it as a refuge for English Catholics.
Calvert died in 1632 and was succeeded by his son, Cecil, who became the second Lord Baltimore. Cecil Calvert led the first group of colonists to Maryland in 1634 and founded the colony's first settlement, St. Mary's City.
Caecilius Calvert is remembered as one of the most important figures in the early history of Maryland. He was a visionary leader who helped to establish the colony as a place where religious freedom and tolerance could be practiced.