It derives from Pope John Paul II's Apostolic letter "Tertio Millenio adveniente", proclaiming a jubilee year (Holy Year) for Christians worldwide in 2000 AD (Anno Domine). Briefly it remarks that "Jesus Christ is the same forever".
"JESUS CHRIST IS THE SAME YESTERDAY AND TODAY" (Heb 13:8)
It speaks of His church's mission to carry on the work of Christ, & the Jubllee to be a celebration of His redeeming Incarnation as Son of God, Jesus Christ. Tertium millenium is the 3rd Millennium after Christ.