Power of the Keys
This term originates in the words addressed by Christ to Peter:
"I will give to thee the keys of the kingdom of heaven.
And whatsoever thou shalt bind upon earth, it shall be bound also in heaven; and whatsoever thou shalt loose on earth, it shall be loosed also in heaven."
In the sentence immediately preceding, Christ had promised to Peter alone the primacy of jurisdiction, under the metaphor of a rock; in the above words, He promised Peter the same power under the metaphor of the keys.
The "kingdom of heaven" here means the militant Church of Christ on earth, for although the term may signify the Church of Christ, both militant on earth, and triumphant in heaven, it is evident that here it is a question of that kingdom in which the power of binding and loosing is to be exercised.
Therefore Christ promises to Peter the keys of His kingdom existing on earth.
The handing over of keys, according to both sacred and profane usage, signifies the conferring of power over any object, the keys of which are handed over to another.
In the days of walled cities, the handing over of the keys of a city to the leader of those besieging signified the surrender of jurisdiction over that city to the besieger.
When, therefore, Christ promises that He will give to Peter the keys of the Church, He ipso facto promises him power over the Church.
This power is supreme jurisdiction, the primacy of jurisdiction over the Universal Church of Christ on earth.
The keys of the Universal Church of Christ on earth are promised to Peter without qualification.
Hence, power over the entire Church is promised Peter in this formula of the keys.
This can mean only supreme power.
Therefore, in virtue of this power of the keys, Peter would be able to loose and bind on this earth anything pertaining to the kingdom of heaven; this means that he would have universal, independent, and supreme jurisdiction over the Church.
New Catholic Dictionary