Sometimes we see people make professions of faith, come into the church, get excited about evangelism and other things, only to later renounce the faith. We wonder: Will this happen to us? A Christian is capable of a radical fall, but never of a total and final fall.
Consider Peter and Judas. Peter rejected Christ, as did Judas. Peter denied Christ, as did Judas. But Christ was praying for Peter, while Christ said that Judas was a son of perdition from the beginning. Judas was never truly converted, while Peter was. Thus, Peter returned to Christ after his season of sin and apostasy. Judas never did. Because Christ intercedes for us, we can have confidence that we also will never fully depart from Him.
What makes our regeneration permanent is not our perseverance, but Gods preservation. Ultimately it is not how diligently we persevere, and persevere we must, but how well God preserves us in faith.