Five impossible things must happen before a saved soul can be lost.
Someone has to pluck us out of the hand of God Himself. "My Father, which gave them Me, is greater than all; and no man is able to pluck them out of My Father's hand." John 10:29.
Someone has to break the seal of ownership which made us God's purchased possession. "Ye were sealed with that Holy Spirit of promise, which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of His glory." Eph. 1:13, 14.
Someone has to cast out the indwelling Spirit of God. "Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?" 1 Cor. 3:16. "And I will pray the Father, and He shall give you another Comforter, that He may abide with you forever." John 14:16.
Someone has to separate us from the love of Christ. "If God be for us, who can be against us? He that spared not His own Son, but delivered Him up for us all, how shall He not with Him also freely give us all things? Who shall lay anything to the charge of God's elect? It is God that justifieth. Who is he that condemneth? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also maketh intercession for us. Who shall separate us from the love of Christ? shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword? As it is written, For Thy sake we are killed all the day long; we are accounted as sheep for the slaughter. Nay, in all these things we are more than conquerors through Him that loved us. For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord." Rom. 8:31-39.
5) Someone has to erase the believer's name from the Lamb's Book of Life. "Rejoice, because your names are written in heaven." Luke 10:20. "Those... whose names are in the book of life." Phil. 4:3. "Whosoever was not found written in the book of life was cast into the lake of fire." Rev. 20:15. "They which are written in the Lamb's book of life." Rev. 21:27.
Our names, from the palms of His hands
Eternity will not erase;
Impressed on His heart they remain,
In marks of indelible grace;
And we to the end shall endure,
As sure as the earnest is given;
More happy, but not more secure,
The spirits departed to heaven.