Perseverance is due to the purpose of God [in saving men and thereby bringing glory to his name], to the work of Christ [in cancelling men's debt and earning their righteousness ], to the indwelling of the Holy Spirit [in sealing men in salvation and leading them in God's ways], and to the primal source of all, the infinite, mysterious, and immutable love of God.

On a practical level, Christians do not claim to know who is elect and who is not, and the only guide they have are the verbal testimony and good works (or "fruit") of each individual. Any who "fall away" (that is, do not persevere unto death) must not have been truly converted to begin with, though Christians don't claim to know with certainty who did and who did not persevere.

John 6:37-40 "All that the Father gives me will come to me, and whoever comes to me I will never cast out. For I have come down from heaven, not to do my own will but the will of him who sent me. And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day. For this is the will of my Father, that everyone who looks on the Son and believes in him should have eternal life, and I will raise him up on the last day."