What is the source of this supreme joy?

It is the contemplation of the light of God, and of His glory, and union with Him. The apostle Paul says, “For now we see in a mirror, dimly, but then face to face. Now I know in part, but then I shall know just as I also am known” (1 Cor. 13:12) . And our Lord Christ says, “Then the righteous will shine forth as the sun in the kingdom of their Father” (Matt. 13:43).