God's guarantee for believers in Christ is salvation made sure. Faith stands upon the covenant promise and oath of God in Christ. He is the believer's surety (Hebrews 7:22). Salvation is sure based upon the life, death, resurrection, ascension, and return of our Lord Jesus Christ. He is the believer's surety of salvation through the everlasting covenant.
Salvation is made sure in Christ through the Holy Spirit. He convinced you to receive Christ by faith (John 16:7-11). The Holy Spirit produced the new birth in your heart as a believer in Christ (John 3:7-8). Also, the Holy Spirit sealed you upon trusting in Christ (Ephesians 1:13). He not only began the good work in you, but He continues until completion (Philippians 1:6). The Holy Spirit is your guarantee.
Salvation is made sure through the purchase of Christ. He died to purchase a people unto God. As a believer in Jesus, you are bought with a price, and that price is His precious blood (1 Corinthians 6:19-20). God's children are bought through the blood of the everlasting covenant (Hebrews 13:20-21).
Salvation is made sure until redemption is completed when Jesus comes again. "You were sealed for the day of redemption" (Ephesians 4:30). In that day, believers shall be glorified into the likeness of Jesus. We shall be "glorified together" (Romans 8:17). We are "eagerly waiting for the adoption, the redemption of the body" (Romans 8:23).
Salvation is made sure to the praise of God's glory. God saved us in Christ to the praise of His glory. He saves and keeps us to the praise of His glory. Three times in chapter one of Ephesians, we are told that God saves us to the praise of His glory (Ephesians 1:6, 12, 14). Those whom He saves, "He is able to keep" (2 Timothy 1:12). God gets all the praise, honor, and glory for our salvation in Christ.