Romans 8:17

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 with him. [In interpreting this passage we should remember that Paul is speaking to those already converted. Hence, in these and in the preceding verses, he is not telling them how to become children of God, but how to continue such. Now, it is true that the Spirit lays down the terms by which we may become Christians, and if we obey these terms, then both the Holy Spirit and our own spirits testify that we are sons of God. But since Paul is not addressing converts, such an interpretation would be wide of his thought, which is this: If the Holy Spirit indeed leads us in a conflict with sin and a steady effort towards righteousness, and if we submit to be thus led, then the Holy Spirit unites with our spirit to testify that we are God's children. The testimony is, of course, self-directed. i. e., the testimony is for the purpose of assuring and confirming our own faith. If we are led, we know it, and so our own spirit testifies to us. If we are led, in the godly, spiritual path, it can be none other than the Holy Spirit who leads; and so, in the very act of leading, the Spirit testifies to us. And, lastly, if we are led, and if we follow, this union of our spirit and God's Spirit in joint action proves us children of God; for our co-operation with God in this paternal government of his shows us accepted of him as his children. But we can not be children in this one respect of government without being children also in the other respect of heirship. We are, therefore, God's heirs, joint-heirs with his only begotten Son, provided that we are truly led of the Spirit as he was, which we may readily test, for the Spirit led him through suffering to glory, and should lead us by the same pathway, if we are to enjoy somewhat of the same glory.]