Jesus Christ is the perfect image – the exact likeness – of God and is in the very form of God (Phil. 2:6), and has been so from all eternity. By describing Jesus in this manner, Paul emphasizes that He is both the representation and manifestation of God. Thus, He is fully God in every way (John 8:58; 10:30-33; Heb 1:8). Firstborn in this context means highest in rank, not first created.
Commentary from the MacArthur study Bible, notes for Colossians 1:15.