Keys of the kingdom: That which unlocks the kingdom of God so that humans may enter. The proclamation of the message that Jesus Christ is the Son of God; the confession that Jesus Christ is the Son of God. [Matthew 16:15-20]
King: the one in authority. In the Bible God [and Christ] is the ultimate King.
Kingdom of God, kingdom of heaven, kingdom of Christ:  God’s divine authority;  the reign of God [or Christ] in the hearts and lives of human beings;  the area or sphere in and over which God or Christ reigns;  the ultimate kingdom after Satan, sin and death are finally terminated.