The Greek word "huiothesia" is translated as adoption. That word signifies the place and condition of a son given to one whom it does not naturally belong.
We were not begotten of the father as Jesus was: "For God so loved the world, that he gave his ONLY BEGOTTEN SON, that whosoever believeth in him should not perish, but have everlasting life"(John 3:16 KJV).
Christ is the only begotten son. For anyone to be part of that family, it has to be by adoption. Adamic nature and sin puts us in a different family not because we chose to. This is due to the effect of the fall of Adam.
God desired a family relationship with mankind from the beginning but sin altered that plan and we became estranged from God. In His mercy, He provided a way out of the problem sin created through Christ's sacrifice and He opened the door of sonship for us through adoption so that we can enjoy the rights and privileges of sons: "For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father" (Romans 8:15 KJV).
Another wonder of God's marvelous grace to those of us in Christ is our adoption as sons. I can boldly call God my father because I'm adopted into His family and I have rights and privileges of sons.
This( adoption) is God's plan from before time began: "having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will"( Ephesians 1:5 KJV).
Religion cannot understand this because religion sees God as God only but not as a father, a loving one at that. It is only in the Christian faith God is called a father.
However, adoption as sons doesn't end at the cross when we came to Christ. There's another phase of adoption we're yet to experience. It is still in the future: "Not only so, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for our adoption to sonship, the redemption of our bodies"( Romans 8:23 NIV).
You'll notice this adoption is yet to happen. Paul said "we are eagerly waiting for our adoption to sonship". This adoption will happen when Christ comes. The same Paul told us we've been adopted as sons.
Is Paul contradicting himself? Of course not. It seems to me that most of the things wrought on the cross and given to us still has a futuristic side to it. It is not a fait accompli.
It is like this because God want us to be forward-looking believers whose gaze will be fixed on heaven- our home. The early church had this understanding that's why they're a forward-looking church that was eager for the coming of Jesus.
That has changed today as many believers have this earth as their focus. They live for now and now alone. At the appearing of Christ, we shall enter into the perfection of all things. Are you looking forward to His coming?
Father, I receive grace to walk my pilgrim journey faithfully in Jesus name.