The answer to the question of whether Old Testament believers in the Lord had eternal security is the same as the answer to whether New Testament believers have eternal security. God does not change (Numbers 23:19; Malachi 3:6) and is therefore consistent regarding His offer of salvation. If the New Testament believer is eternally secure in his or her salvation, so is the Old Testament believer.
Salvation has always been a gift of God, by grace through faith (Genesis 15:6; Romans 4:1–8; Ephesians 2:8–9). In the Old Testament, people were responsible to exercise faith in what God had revealed to them and to trust in God for their salvation; their faith was made evident in their actions. In the New Testament era, we are responsible to exercise faith in what God has done through Christ and to trust in God for our salvation; our faith is made evident in our actions.
The gift of God is eternal life, not temporary life (Romans 6:23). God’s offer of salvation is not that we “might” have eternal life if we try really hard or if we don’t mess up too much. Salvation is based on what Christ has done, not on what we have done or will do. Hebrews 11 gives many examples of the faith of Old Testament believers, including those who sinned greatly. It confirms that they will receive what God has promised, based on the work of Christ (Hebrews 11:39–40).
Romans 8:38–39 confirms the eternal security of all believers in the strongest terms: “For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.” In this passage Paul goes out of his way to assure God’s children that absolutely nothing at all can destroy their relationship with the God of their salvation.
One major difference between believers in the Old Testament and believers in the New Testament relates to the role of the Holy Spirit. In the New Testament, the Holy Spirit enters a believer at the point of salvation and permanently indwells him (1 Corinthians 12:13; Ephesians 1:13–14). This indwelling is something that Jesus had promised would occur after His ascension to heaven (John 14:17; 16:7; cf. John 7:39). In the Old Testament, it seems that the Spirit of God did not permanently indwell believers; rather, the Spirit came upon them from time to time to accomplish God’s purposes through them (Judges 3:10; 14:19; 1 Samuel 10:10; 16:14; Psalm 51:11).
The fact that the Holy Spirit came and went does not mean Old Testament believers lost (and regained) their salvation. The context of Psalm 51 is David’s repentant prayer following his sin with Bathsheba. The infant born of David and Bathsheba’s sinful union died, and David said that he would one day go to be with the child (2 Samuel 12:16–23). In other words, David believed he would one day join his departed son in heaven. David did not lose his salvation, despite his sin.
The Old Testament saints, by faith, looked forward to the Messiah who would come to save (John 8:56). They put their faith in God to save them: “Praise be to the Lord, to God our Savior, who daily bears our burdens. Our God is a God who saves; from the Sovereign Lord comes escape from death” (Psalm 68:19–20). Today, we look back, by faith, to the Christ who has come to save. We, too, put our faith in God to save us. Together, saints from the Old and New Testaments trust that God is true to His promises: “He is the Rock, his works are perfect, and all his ways are just. A faithful God who does no wrong, upright and just is he” (Deuteronomy 32:4). Because of His faithfulness, we are eternally secure.