Did Old Testament believers have eternal security?

from the old testament

• Old Testament believers were saved the same way New Testament believers are saved today: by faith in God. Of Abraham, Genesis says, "And he believed the LORD, and he counted it to him as righteousness" (Genesis 15:6). Abraham did not have to follow the Law perfectly—in fact, the Law had not yet been given—but God had made a covenant with Abraham, and Abraham had faith that God would keep His covenant.
• God’s steadfast love and covenant faithfulness assured His people that He would not forsake those who truly trusted Him (Deuteronomy 7:9; Psalm 103:17–18; Psalm 37:28).
• God is the same yesterday, today, and tomorrow (Numbers 23:19; Malachi 3:6). He rewards those who genuinely seek Him. Old Testament believers had the same kind of faith as ours, and they placed this faith in the same God that we do, and by His grace, He wrote their names in the book of life.
• Other than our knowledge of the revealed mystery of Jesus's atoning work on the cross, the main difference between Old Testament believers and believers after Jesus's death is our access to the Holy Spirit. Old Testament believers did not have the Holy Spirit permanently living in them; rather, the Holy Spirit would rest upon them at various times (Judges 3:10; 1 Samuel 10:10; 16:14; Psalm 51:11). We now have the benefit of God's Spirit living in us, continually providing wisdom, truth, conviction, help, and intercession (see John 16:7–14).
• The Old Testament pointed to eternal life for the faithful, showing that they looked forward to a secure, everlasting future with God (Daniel 12:2).

from the new testament

• People in the Old Testament were saved the same way people are saved after Christ's death and resurrection: by grace through faith (Ephesians 2:8–9). The quality of that salvation is the same—God gives eternal life. A person cannot be saved one moment and not saved the next.
• Just as New Testament believers have eternal security in Jesus Christ, so do Old Testament believers who put their faith in God. Their faith, and ours, did not rest in one who changes, but in the God who never changes (James 1:17).
• Hebrews 11 highlights Old Testament believers who lived by faith, looking ahead to God's promises. Though they didn’t see Christ’s fulfillment, they longed for a heavenly home, and God honored their faith by preparing a city for them (Hebrews 11:13–16).
• Old Testament believers were also saved by grace through faith because "without faith it is impossible to please [God]" (Hebrews 11:6).
• Hebrews also gives us insight to Moses's salvation: "By faith Moses, when he was grown up, refused to be called the son of Pharaoh's daughter, choosing rather to be mistreated with the people of God than to enjoy the fleeting pleasures of sin. He considered the reproach of Christ greater wealth than the treasures of Egypt, for he was looking to the reward" (Hebrews 11:24–26). Moses did not know about Christ's work on the cross, but he did know who God was and saw Him as more valuable than the pleasures of Egypt. Moses sacrificed the sin in his life for the greater reward that rests in following God, and in doing so, he joined his life to Christ's in his suffering.
• The actions of Old Testament believers demonstrated their faith, just as our actions demonstrate our faith (Ephesians 2:10; James 2:18; Philippians 2:12–13).

implications for today

Old Testament believers trusted God's promise to provide a Savior; today we trust that He has provided that Savior. In either case, salvation is made possible through Jesus's work on the cross. Jesus provides complete atonement for sin (Hebrews 10). It is only by Him that we are saved, and when we are saved, we are saved forever. God does not revoke His gift of salvation; all who are saved, no matter in what era they were saved, are eternally secure.