The Keys of Heaven
Two crossed keys stand for the authority Christ gave St. Peter: "I will give you the keys of the kingdom of heaven" (Matthew 16:19).
What the Keys of Heaven means
The keys recall Christ's promise to Peter: "I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven" (Matthew 16:19). The Catechism explains that the "power of the keys" designates authority to govern the house of God, which is the Church, and that the power to bind and loose covers absolving sins, pronouncing doctrinal judgments and making disciplinary decisions (CCC 553).
The image has Old Testament roots. In Isaiah 22, "the key of the house of David" is given to Eliakim as a sign of full authority in the kingdom of Judah, and the Catholic Encyclopedia reads Christ's promise to Peter against that background.
Two crossed keys became St. Peter's emblem, the first emblem assigned to any saint, at about the same time the sword came to mark St. Paul. They appear in papal coats of arms and in the arms of Vatican City State, which show two keys crossed, one gold and one silver, under the papal tiara on a red field.
