Whatever apostasy occurs in Christianity, it may never prompt us to question the unchanging faithfulness of God, the certainty of his counsel, the enduring character of his covenant, or the trustworthiness of his promises. One should sooner abandon all creatures than fail to trust his word. And that word in its totality is one immensely rich promise to the heirs of the kingdom.
It is not just a handful of texts that teach the perseverance of the saints: the entire gospel sustains and confirms it. The Father has chosen them before the foundation of the world (Eph. 1:4), ordained them to eternal life (Acts 13:48), to be conformed to the image of his Son (Rom. 8:29). This election stands (Rom. 9:11; Heb. 6:17) and in due time carries with it the calling and justification and glorification (Rom. 8:30).
Christ, in whom all the promises of God are Yes and Amen (2 Cor. 1:20), died for those who were given him by the Father (John 17:6, 12) in order that he might give them eternal life and not lose a single one of them (6:40; 17:2); he therefore gives them eternal life and they will never be lost in all eternity; no one will snatch them out of his hand (6:39; 10:28). The Holy Spirit who regenerates them remains eternally with them (14:16) and seals them for the day of redemption (Eph. 1:13; 4:30).
The covenant of grace is firm and confirmed with an oath (Heb. 6:16-18; 13:20), unbreakable like a marriage (Eph. 5:31-32), like a testament (Heb. 9:17), and by virtue of that covenant, God calls his elect. He inscribes the law upon their inmost being, puts his fear in their heart (Heb. 8:10; 10:14ff.), will not let them be tempted beyond their strength (1 Cor. 10:13), confirms and completes the good work he has begun in them (1 Cor. 1:9; Phil 1:6), and keeps them for the return of Christ to receive the heavenly inheritance (1 Thess. 5:23; 2 Thess. 3:3; 1 Peter 1:4-5).
In his intercession before the Father, Christ acts in such a way that their faith may not fail (Luke 22:32), that in the world they may be kept from the evil one (John 17:11, 20), that they may be saved for all times (Heb. 7:20), he is to behold his glory (John 117:24). The benefits of Christ, which the Holy Spirit imparts to them, are all irrevocable (Rom. 11:29). Those who are called are also glorified (8:30). Those who are adopted as children are heirs of eternal life (8:17; Gal. 4:7). Those who believe have eternal life already here and now (John 3:16). That life itself, being eternal, cannot be lost. It cannot die since it cannot sin (1 John 3:9). Faith is a firm ground (Heb. 11:1), hope is an anchor (6:19) and does not disappoint us (Rom. 5:5), and love never ends (1 Cor. 13:8).
Excerpted from Reformed Dogmatics 4.269-70 (paragraph breaks are mine)