In Romans 8:15–17, Paul was writing to Christians about what was available to them through their faith in Christ.
For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him [the Holy Spirit] we cry, “Abba, Father.” The Spirit himself testifies with our spirit that we are God’s children. Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory. (niv)
The word Abba is the Aramaic or Hebrew equivalent of the English word “Daddy.” Thus, we see here a relationship of intimacy with God the Father, whom we address as Daddy. The Spirit of God Himself gives us the assurance and confidence to do so.
The Bible tells us that we are God’s children, but the Spirit of God reinforces that truth personally to each of us, in our hearts. We are God’s children. And when we become children, we become heirs. We are made heirs of God and coheirs with Christ. Of course, one condition is stated: if we are willing to share His sufferings. If we share in the inheritance, we also share in the sufferings. Remember, the pearl is a product of pressure and stress.
It is important to understand what it means to be coheirs. It does not mean that we each get a little fraction of the total inheritance. Rather, it means that Jesus, as the first Son of God, has the whole inheritance, and we share the whole inheritance with Him. Each one of us has a right to the entire inheritance, which is the inheritance of Jesus. The law of God’s kingdom is sharing. We do not each grab our own portions, but we all share together everything that God the Father has and everything that Christ the Son has.