Question: "What is an heir in the Bible?"
Answer: The literal meaning of heir is “someone who has been appointed to receive an inheritance.” An heir is a person who receives something of value from a father (see Mark 12:7). The Bible sometimes uses the word heir to describe us as recipients of a gift from God (Galatians 4:7). We are “heirs” of God, and Christ is our “co-heir” (Romans 8:17).
The heirs most often mentioned in the Old Testament were sons, principally the firstborn, born only to legitimate wives, not concubines. A firstborn son held the birthright, which means he was heir to a larger portion than what his brothers received (Deuteronomy 21:17; Genesis 21:10). There are a few instances when a younger son became the chief heir (1 Kings 1:13; Genesis 25:23). The patriarch of the family—or God Himself, in the case of Jacob and Esau—had the right to choose the chief heir and could set aside the normal rights of the firstborn. For example, Jacob gave Reuben’s birthright to Joseph’s sons because of Reuben’s sin (1 Chronicles 5:1). An heir could receive his inheritance as a lump-sum payment during his father’s lifetime in lieu of waiting until his father’s death (see Luke 15:12). Daughters only became heirs if they had no brothers, and only if they married within their own tribe (Numbers 27:8; 36:6).
The New Testament portrays Jesus Christ as God’s “firstborn” Son (Hebrews 1:6, see Psalm 89:27); that is, Jesus holds the “birthright” and is the heir of all the Father has (Hebrews 1:2). God has given Christ all of creation as a gift, because all was created for Him (Colossians 1:16; John 3:35). The inheritance that Christ receives includes believers, those whom the Father has given to the Son (John 6:37).
Believers are co-heirs with Christ (Romans 8:17). Meaning, believers have been given the privilege of sharing Christ’s inheritance. As adopted sons of God, Christians are treated as firstborn heirs (Hebrews 12:23). Our inheritance includes salvation (Hebrews 1:14), eternal life (1 Peter 1:4), and even a measure of the throne of Christ (Revelation 3:21).
The heirs of God’s promises receive their inheritance through faith: “It was not through the law that Abraham and his offspring received the promise that he would be heir of the world, but through the righteousness that comes by faith” (Romans 4:13; Galatians 3:6). The promise given to Abraham concerning his “offspring” (Genesis 22:17–18) was a promise of Jesus the Messiah (Galatians 3:16). So an heir of God is someone who, through faith, receives Christ, who fulfilled the Abrahamic promise of being a blessing to all nations (Galatians 3:29; Revelation 21:24–27). To all who believe is given the Holy Spirit as “a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory” (Ephesians 1:14).