There’s a lot of different perspectives in Christianity about the level of assurance a believer can have. D.A. Carson, of Trinity Evangelical Divinity School, offers some great insight:
For non-reformed theologies…”at the end of the day, the security of the believer finally rests with the believer. For those in the opposite camp [Reformed], the security of the believer finally rests with God–and that, I suggest, rightly taught, draws the believer back to God himself, to trust in God, to a renewed faith that is of a piece with trusting him in the first place.”
When people believe that their free-choice brought them to Christ, that their works bring about sanctification, and that their determination keeps them enduring, instead of God’s election, grace, mercy, and power, they are ultimately trusting in themselves and not God. How arrogant and self-centered!
Furthermore, if we could lose our salvation, that would mean that the Christian could perform a work that is more powerful than Almighty God by leaving his saving grasp. We must remember what Jesus said in John 10:27-29, “My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father’s hand.”
Take joy in that promise. Rejoice that your Savior is not weak or incapable of preserving you. He is gloriously powerful and more than able to hold you in his grip for all eternity.