When people come to know Christ as their Savior, they are brought into a relationship with God that guarantees their eternal security. Jude 24 declares, "To Him who is able to keep you from falling and to present you before His glorious presence without fault and with great joy." God's power is able to keep the believer from falling. It is up to Him, not us, to present us before His glorious presence. Our eternal security is a result of God keeping us, not us maintaining our own salvation.
The Lord Jesus Christ proclaimed, "I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all, no one can snatch them out of my Father's hand" (John 10:28-29b). Both Jesus and the Father have us firmly grasped in their hand. Who could possibly separate us from the grip of both the Father and the Son?
Ephesians 4:30 tells us that believers are "sealed for the day of redemption." If believers did not have eternal security, the sealing could not truly be unto the day of redemption, but only to the day of sinning, apostasy, or disbelief. John 3:15-16 tells us that whoever believes in Jesus Christ will "have eternal life." If a person were to be promised eternal life, but then have it taken away, it was never "eternal" to begin with. If eternal security is not true, the promises of eternal life in the Bible would be in error.
The most powerful argument for eternal security is Romans 8:38-39, "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." Our eternal security is based on God's love for those whom He has redeemed. Our eternal security is purchased by Christ, promised by the Father, and sealed by the Holy Spirit.