Essentially Paul was saying to the Corinthians, “You have believed on Jesus Christ through my ministry. The gospel came to you in power through the Spirit of God, and this attests to the authenticity of my apostleship and the genuineness of the gospel I preached.” (It therefore follows that men are only saved by the preaching of the true gospel by true ministers of Jesus Christ; Men are not saved by listening to false prophets who preach a false gospel.)
Christ is the True Witness (Rev 3:14). He has the Father’s seal. The Father's seal is the Spirit of God. (See Acts 10:38; Luke 3:21-22; Isaiah 61:1-3; John 1:32.) It was the Spirit of God that raised Christ from the dead. "But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you." Romans 8:11. Christ's seal attests to Him being sent by God and approved by God in this: He was raised from the dead by the glory of the Father; His people are saved from their sins; their eyes are opened; they are turned from darkness to light; they are turned from the power of Satan unto God (Romans 1:4; Isaiah 42:1-7; Acts 26:18)
The scripture says that, “God was in Christ, reconciling the world unto Himself (2 Cor 5:19).” Reconciliation is ascribed to God in Christ. It is God who has reconciled us to Himself by Jesus Christ. This inseparable work of God the Father, the Holy Spirit and the Lord Jesus Christ is included in what is meant by the phrase, “Him hath God the Father sealed.” God the Father was in Christ, God the Spirit was in Christ, the Lord Jesus is the Son of God, and therefore God was in Christ, reconciling the world unto Himself, not imputing their sins to them.
We see the Father, Son and Holy Spirit in Christ's seal throughout scripture. It was the Father's will expressed in the volume of the book that Christ would redeem His people from their sins. The Father's will was breathed out by God the Holy Spirit and is recorded in scripture. That will of the Father regarded God's Son who would fulfill all shadowy sacrifices and give pleasure to God for His once for all satisfaction for the sins of His people. Thus, it was the Father's will, the anointing of the Spirit, and the work of the Lord Jesus Christ in our salvation where the seal of Christ is seen in its full breadth and scope. The triune God was in Christ. The Lord Jesus Christ died to pay the debt that His people's sins incurred to God's justice according to the will of God. His death paid their debt, setting them free from the curse of the law, and reconciling them to God by appeasing God's wrath, all according to His eternal will recorded in scripture breathed out by His own Holy Spirit. And now, the Lord Jesus reigns in glory and intercedes according to the Father's will and sends forth His Spirit to bring about to His own elect people the benefits of that salvation. See Hebrews 9:15; Psalm 40; Hebrews 10:7; Galatians 3:13; Romans 3:25; Acts 2:22-33ff.
If it is Christ that died (Romans 8:34), if God the Father sealed Him for this purpose (John 6:27), if He through the eternal Spirit offered Himself without spot to God (Hebrews 9:14), if it was God in Christ reconciling the world of His elect to Himself, then for Him to fail would be for God's seal to fail, for God Himself to fail. Christ fails if anyone for whom He died perishes. Everyone for whom Christ died must and shall be saved, must and shall be presented faultless and blameless before the throne of His glory with exceeding joy, or God's seal is made null and void, and that cannot happen.
-- Rick Warta