In the Old Testament the firstborn son had authority in the household when the father was absent, and on the death of his father he inherited twice as much as the other sons. (2 Kings 2:9)
In the New Testament Jesus Christ is described as the firstborn, not only because he was the eldest in his family, but also in highly significant spiritual ways. In Romans 8:29 he is described as ‘the firstborn among many brothers’. He has the highest rank in the family of believers and has authority over them.
In Colossians 1:18 Jesus is called ‘the beginning, the firstborn from the dead’, even though there were others, such as Lazarus, who had been miraculously raised from the dead before he was. The difference was that the earlier examples were raised to mortality again, whereas Jesus Christ was the first to be raised to eternal life. He is the firstborn because the faithful will also be rewarded with eternal life when Jesus Christ returns to the earth again. In Colossians 1:15 he is called ‘the firstborn of every creature’ because he has been given authority over all things, as will be clearly demonstrated when the Kingdom of God is establishes on the earth.
In Hebrews 12:23 the believers are referred to as ‘the firstborn’, since they are in a privileged position in God’s sight and they have the responsibility of exercising the same values and objectives as their Father.