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." (John 10:28-29)
Who has the Father given to Christ? All who believe in Him.
As John MacArthur said, "If you could lose your salvation, you would." In fact, nothing in all creation could cause you to lose your salvation. "They will never perish." (from John 10:28)
For I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord. (Romans 8:38-39)
And finally, what could be better than a guarantee of your salvation from God, the Creator of all things, who designed salvation from before the ages?
And it is God who establishes us with you in Christ, and has anointed us, and who has also put his seal on us and given us his Spirit in our hearts as a guarantee. (2 Corinthians 1:21-22)
He who has prepared us for this very thing is God, who has given us the Spirit as a guarantee. (2 Corinthians 5:5)
In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory. (Ephesians 1:13-14)
Remember, salvation is a gift, not of works. It is by grace one is saved, through faith (trusting God instead of yourself for your soul salvation). Works cannot earn salvation, nor can they maintain salvation. Christ did the work, all of it.
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. (Ephesians 2:8-9)
Once saved always saved? Absolutely! Guaranteed!
(See previous post on What's Become of You in Christ?)