Why are Christians joint heirs with Christ?
Christians are joint heirs with Christ because the Spirit of God leads them and bears witness with their spirit that they are already the children of God. As children of God, they are heirs, just as a son is heir of his father’s estate, making them joint heirs with Christ.
Les explains that the secret of the Christian life is not trying to make points with the Almighty or merit His favor, but responding to what He has done for us and through us. He points to Romans 8:14, where as many as are led by the Spirit of God are the sons of God. Then verse 16 says the Spirit Himself bears witness with our spirit that we are already the children of God. The next verse declares that if we are children, then naturally we are heirs—just as sure as a son is the heir of the father and mother's estate—so we now are joint heirs with Christ. This inheritance flows from being children of God through the Spirit’s witness, not from human effort.
And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.