3 times 7 equals 21
3 is a very important number in Christianity, such as the Trinity of God, Jesus Christ was resurrected on the third day after his death, and there are three heavens in heaven. Asia has 3 sons and so on.
7 is also a very important number. For example, there are 7 days in a week, 7 churches in Revelation, and several 7s are often used to calculate time in prophetic books. God rests on the 7th day. 7 represents God’s perfection, and there are 7 plagues and 7 bowls in Revelation.
So the 21 rings are to commemorate Christ and show that they believe in Christianity. Do you understand?