Why are believers heirs of God and joint heirs with Christ?
Through the Spirit of adoption, believers become full children of God, and children are heirs—God is their inheritance, and they share Christ's inheritance. At salvation God declares them full sons, co-equal with Christ, so everything that belongs to Christ will one day be theirs.
Les teaches that Paul says believers have been placed through the Spirit of Adoption as complete heirs with the Father. In verse 16, the Spirit Himself bears witness with our spirit that we are the children of God; if children, then heirs, heirs of God, and joint heirs with Christ. Les explains that the average believer has no hint of this: at the moment of salvation, when we believed, God declared us a full son of the Father and a joint heir with Christ. Les says this is beyond comprehension. He illustrates joint heirship by husband and wife being co-equal, so what is his is hers and vice versa. Therefore, one day everything that is Christ's will be ours; everything that is His is going to be ours, because the Book says it.