“The Bible teaches that [the] eternal God has always had

• a perfect image of himself (Colossians 1:15),
• a perfect radiance of his essence (Hebrews 1:3),
• a perfect stamp or imprint of his nature (Hebrews 1:3),
• a perfect form or expression of his glory (Philippians 2:6).

We are on the brink of the ineffable here, but perhaps we may dare to say this much: as long as God has been God, he has been conscious of himself, and the image that he has of himself is so perfect and so complete and full as to be the living, personal reproduction (or begetting) of himself. And this living, personal image or reflection or form of God is God, namely, God the Son. And therefore God the Son is co-eternal with God the Father and equal in essence and glory.

-John Piper, The Pleasures of God: Meditations on God’s Delight in Being God, 28.