John Newton said of this truth: “Jesus, to whom I have been led to commit myself, has engaged to save me, absolutely, and from first to last. He has promised not only that He will not depart from me—but that He will put, keep, and maintain His fear in my heart—so that I shall never finally depart from Him! And if He does not do this for me—I have no security against my turning apostate! For I am so weak, inconsistent, and sinful; I am so encompassed with deadly snares from the world; and I am so liable to such assaults from the subtlety, vigilance, and power of Satan—that, unless I am ‘kept by the power of God,’ I am sure I cannot endure to the end! In short, I must sit down in despair—if I did not believe that He who has begun a good work in me, will carry it out to completion.”
In thinking about the believer's security, and about the call to perseverance, I believe the Westminster confession, chapter 17, puts it together well...
I. They, whom God has 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.
II. This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father; upon the efficacy of the merit and intercession of Jesus Christ, the abiding of the Spirit, and of the seed of God within them, and the nature of the covenant of grace: from all which arises also the certainty and infallibility thereof.
III. Nevertheless, they may, through the temptations of Satan and of the world, the prevalence 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.