Saved to the Uttermost By John MacArthur

All true believers will be saved to the uttermost. Christ’s High Priestly ministry guarantees it. They have been justified, they are being sanctified, and they will be glorified. Not one of them will miss out on any stage of the process, though in this life they all find themselves at different points along the way. The truth has been known historically as the perseverance of the saints.
Perseverance means that “those who have true faith can lose that faith neither totally nor finally” (Anthony A. Hoekema, Saved by Grace, 234). It echoes God’s promise through Jeremiah: “I will make an everlasting covenant with them that I will not turn away from them, to do them good; and I will pity the fear of Me in their hearts so that they will not turn away from Me” (32:40, emphasis added).
The Westminster Confession of Faith has defined perseverance as follows:
They whom God hath accepted in His Beloved, effectually called and sanctified by his Spirit, can neither totally nor finally fall away from the state of grace; but shall certainly persevere therein to the end, and be eternally saved (chap. 17, sec. 1).
This definition does not deny the possibility of miserable failings in one’s Christian experience, because the Confession also said,
Nevertheless [believers] may, through the temptations of Satan and of the world, the prevalency of corruption remaining in them, and the neglect of the means of their preservation, fall into grievous sins; and for a time continue therein; whereby they incur God’s displeasure, and grieve his Holy Spirit: come to be deprived of some measure of their graces and comforts; have their hearts hardened, and their consciences wounded; hurt and scandalize others, and bring temporal judgments upon themselves (sec. 3).
Sin is a reality in the believer’s experience, so it is clear that insistence on the salvific necessity of a working faith does not include the idea of perfectionism. Scripture is filled with warnings to people in the church lest they should fall away (cf. Heb 6:4–8; 1 Tim 1:18–19; 2 Tim 2:16–19). But those warning passages do not negate the many promises that believers will persevere:
Whoever drinks of the water that I shall give him shall never thirst; but the water that I shall give him shall become in him a well of water springing up to eternal life (John 4:14).
I am the bread of life; he who comes to Me shall not hunger, and he who believes in Me shall never thirst (John 6:35).
You are not lacking in any gift, awaiting eagerly the revelation of our Lord Jesus Christ, who shall also confirm you in the end, blameless in the day of our Lord Jesus Christ. God is faithful, through whom you were called into fellowship with His Son, Jesus Christ our Lord (1 Cor 1:7–9).
May the God of peace Himself sanctify you entirely: and may your spirit and soul and body be preserved complete, without blame at the coming of our Lord Jesus Christ. Faithful is He who calls you, and He also will bring it to pass (1 Thess 5:23–24).
They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, in order that it might be shown that they all are not of us (1 John 2:19).
Now unto him that is able to keep you from falling, and to present you faultless before the presence of his glory with exceeding joy, to the only wise God our Saviour, be glory and majesty, dominion and power, both now and for ever. Amen (Jude 24–25, KJV, emphasis added in all citations).
Horne observed,
It is noteworthy that when Jude exhorts us to keep ourselves in the love of God (v. 21) he concludes with a doxology for Him who is able to keep us from falling and who will present us without blemish before the presence of His glory (v. 24). The warning passages are means which God uses in our life to accomplish His purpose in grace. (Charles Horne, Salvation, 95)
And, it could be added, the warning passages like Jude 21 reveal that the writers of Scripture were very keen to alert those whose hope of salvation might be grounded in a spurious faith. Obviously the apostolic authors were not laboring under the illusion that every person in the churches to whom they were writing was genuinely converted.