Andrew Murray, in The Holiest of All, writes
"[Jesus] is the outshining of God’s glory, and the express image of His substance. As we only know the sun by the light that shines from it, so is Christ the outshining, the revelation of God’s glory. As the light that shines from the sun is of the same nature with it, so the Son is of one nature with the Father – God of God. And as a son bears the likeness of his father, because he has his life and nature from him, so the Son of God is the express image of His substance. He is of one substance with the Father – its express image – and hath therefore life in Himself, even as the Father hath life in Himself.
Someone may be tempted to think that these are theological mysteries too deep for the ordinary Christian, and not needful for our Christian faith and life. And they are inclined to ask, of what importance it can be to a simple believer to know all this? My brother, think not thus. It is all important that we know the glory of Jesus. The more the soul is filled with that glory, and worships Him in it, the more it will see with what confidence it can count upon Him to do a divine and supernatural work in us, and to lead us to an actual living fellowship with God as our Father."