To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory: Colossians 1:27
In the Bible lands and times mirrors were made of polished golden metals. When a person looked in the polished metal he could see himself. But when another person saw the reflection of the person looking in the mirror he would see a glint of gold on their face and their face was brilliant!
When we focus on the Christ in us we see the brilliant glory of the spirit of God in us and we are changed into the same image reflecting brighter and brighter!