"The Spirit himself beareth witness with our spirit, that we are children of God: 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.... And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ" (Rom. 8:16-17; Gal. 4:6-7).
The Greek word kleronomos (one who obtains a lot or portion; lit. to possess a lot) is translated by our English word "heir." Webster defines an "heir" as "one who inherits, or is entitled to inherit."
All of the children of God are His heirs: "if children, then heirs." Paul tells us how we become children and heirs: "For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise" (Gal. 3:26-29). All of God's children are in His family, the church, which is the house(hold) of God (1 Tim. 3:15; Heb. 3:6; 10:21; Gal. 6:10). There are no children of God, no heirs of God, outside of God's family, the church of Christ and of God to which the saved are added by the Lord Himself (Acts 2:38-41, 47; Mark 16:16; Col. 1:13; Eph. 5:5,23; 1 Cor. 12:13).
The Holy Spirit bears witness with our spirit that we are the sons and heirs of God. He bears witness by revealing all of God's truth through the inspired men in the New Testament (1 Jn. 5:5-13; 1 Cor. 2:8-13), teaching and telling us how to believe and obey God to become His children (Heb. 5:8-9). Our spirit bears witness by hearing, understanding, believing and obeying from our hearts these truths revealed by the Holy Spirit (Rom. 6:16-18; 1 Pet. 1:22-25; 1 Cor. 15:1-4). "For as many as are led by the Spirit of God, these are sons of God" (Rom. 8:14). We are led by the Spirit, as He tells us in the word of God what God would have us to believe and obey. We follow the Spirit by doing what He instructs us to do.
Every man can know, and does know, by his knowledge of God's will whether or not he has sincerely, willingly, obediently and understandingly, done God's will from the heart, and whether or not he continues to do it as the Holy Spirit teaches. Those who abide in the truth, revealed by the Spirit, continuing in faithfulness, are heirs who have the hope and
promise of an eternal inheritance in heaven.
Guardian of Truth XXIX: 6, p. 172