For if we died with Him, we will also live with Him;
¹² If we endure, we will also reign with Him;
If we deny Him, He will also deny us;
¹³ If we are faithless, He remains faithful, for He cannot deny Himself.

Some isolate verse 13 when it states “If we are faithless” to argue that Christians may lose their faith, but yet remain saved because “He [God] remains faithful”. This simply ignores the prior verses that teach enduring is essential to reign with him. The proponent may argue that verses 11-12 and referring to losing salvific gifts in heaven. This simply ignores that the context is about “salvation which is in Christ Jesus and with it eternal glory”. This is also apparent by their parallels to other Pauline usages of the same phrases and theological motifs. Dr. Köstenberger notes:

 

In light of the numerous Pauline parallels, it’s likely that the saying/ hymn is original to Paul (cf. 1 Corinthians 13; Rom 8: 28– 39). The statement consists of four conditional clauses (εἰ + indicative verb), of which the first two deal with faithful service and the last two deal with disowning Christ, plus a main clause with a future (first three clauses) or present verb (final clause). …

11 For if we died with him, we will also live with him;	εἰ γὰρ συναπεθάνομεν, καὶ συζήσομεν·
12 if we endure, we will also reign with him;	εἰ ὑπομένομεν, καὶ συμβασιλεύσομεν·
if we deny him, he will also deny us;	εἰ ἀρνησόμεθα, κἀκεῖνος ἀρνήσεται ἡμᾶς·
13 if we are faithless, he remains faithful, for he cannot deny himself.	εἰ ἀπιστοῦμεν, ἐκεῖνος πιστὸς μένει, ἀρνήσασθαι γὰρ ἑαυτὸν οὐ δύναται.

The main subject of the four lines of the hymn isn’t identified explicitly, though it can be reasonably inferred that it is Christ (see esp. v. 11: “For if we died with him”). Nevertheless, by way of double entendre, the reference to faithfulness in v. 13 may on a secondary level also extend to God.  As mentioned, the flow in logic is intriguing and defies simplistic characterization, moving from the promise of living and reigning with Christ (“ die . . . live,” “endure . . . reign”), to the severe consequences of denying Christ (“ deny . . . deny”), to an affirmation of Christ’s (and possibly God’s) unshakable faithfulness even in the face of human faithlessness (“ faithless . . . faithful”), which provides a climactic exclamation point, further accentuated by the element of surprise introduced by the unexpected turn in the final affirmation. “For if we died with him, we will also live with him.” The language and thought are thoroughly Pauline (see esp. Rom 6: 8: “Now if we died with Christ, we believe that we will also live with him” [cf. v. 5; Gal 2: 20; see also the verbal parallels with 2 Cor 7: 3]). In view of these parallels, “died with him” likely refers to the believer’s spiritual union with Christ rather than to martyrdom (though readiness to martyrdom may be implied on a secondary level). Similarly, “live with him” doesn’t refer primarily to the believer’s eternal state but to his present possession of spiritual life by virtue of his union with Christ.

“If we endure, we will also reign with him.” Once again the statement coheres with Paul’s teaching elsewhere (see esp. Rom 5: 17; note also the verbal similarity with 1 Cor 4: 8 and the affinity with Jesus’s teaching in Mark 13: 13 // Matt 24: 13; Matt 10: 22 par.). As in the previous statement, the primary reference is to Christians’ present reign with Christ (though a future dimension cannot be ruled out). The need for believers— including Timothy— to endure (ὑπομένω) is the frequent subject of Paul’s instruction (v. 10; 3: 10– 11; 1 Tim 6: 11; Rom 5: 3– 4; 15: 4– 5; 2 Cor 1: 6; 6: 4; 1 Thess 1: 3; 2 Thess 1: 4; 3: 5; Col 1: 11). Job and the prophets are examples of such endurance (Jas 5: 10– 11), which is a mark of Christian maturity (Titus 2: 2; Jas 1: 3– 4, 12; 2 Pet 1: 6).

Köstenberger, Andreas J. . Commentary on 1-2 Timothy and Titus (Kindle Locations 4711-4749). Holman Reference. Kindle Edition.

A better passage with more relevance has been touched upon in a prior article:

They are rather “dogs”, “swine”, etc unclean and have remained in their worldly desires rather than remaining in the knowledge of the Lord given to them when they were pretending Christians in the Church. In fact, if you hold to the authenticity of 2 Peter you should also hold to 1 Peter which states:

³ Blessed be the God and Father of our Lord Jesus Christ! According to his great mercy, he has caused us to be born again to a living hope through the resurrection of Jesus Christ from the dead, ⁴ to an inheritance that is imperishable, undefiled, and unfading, kept in heaven for you, ⁵ who by God’s power are being guarded through faith for a salvation ready to be revealed in the last time. 

This suggests that God has promised to guard Christian until their salvation is to be revealed. Peter assures his readers that they will certainly receive this inheritance. That future salvation or eschatological deliverance will be theirs. The reason for this confidence is that they “are guarded by God’s power.” That is to say, God has promised to preserve the faithful. So, in order for his promise to not occur, is for God to fail to be truthful to his word. Dr. Thomas Schreiner presents an explanation:

The best solution is to say that 2 Peter uses phenomenological language. In other words, Peter uses the language of “Christians” to describe those who fell away because they gave every appearance of being Christians. They confessed Christ as Lord and Savior, were baptized, and joined the church. But the false teachers and some of those they seduced, though still present physically in the church, were no longer considered to be genuine believers by Peter. Nonetheless, he used “Christian” language to describe them, precisely because of their participation in the church, because they gave some evidence initially of genuine faith. Those who had apostatized revealed that they were never truly part of the people of God since remaining true to the faith is one sign that one truly belongs to God. The words of 1 John apply well to what has happened in 2 Peter: “They went out from us, but they did not belong to us; for if they had belonged to us, they would have remained with us. However, they went out so that it might be made clear that none of them belongs to us” (1 John 2: 19). Peter points in the same direction in the illustration of the dog and pig. In the final analysis, those who fell away never really changed their nature. 1767 They remained dogs and pigs inside. 1768 As Hafemann argues at some length their nature was the cause of their actions. 1769 They may have washed up on the outside and appeared to be different, but fundamentally they were dogs and pigs. In other words, they were always unclean; they only seemed to have changed. Perseverance, therefore, is the test of authenticity.

Schreiner, Thomas R.. 1-2 Peter and Jude: The Christian Standard Commentary (Kindle Locations 9167-9180). B&H Publishing Group. Kindle Edition.

https://spirited-tech.com/2021/05/19/can-a-christian-lose-salvation-2/

The point of this is that ultimately our assurance of salvation remains because God is the one that grounds our salvation. The fact he has regenerated us and brought us to faith means that he will ultimately keep us in the faith:

What seems to have happened is that, in a rather typical way (cf., e.g., 1 Cor. 8:3), Paul could not bring himself to finish a sentence as it began. It is possible for us to prove faithless; but Paul could not possibly say that God would then be faithless toward us. Indeed, quite the opposite. If we are faithless (and the context demands this meaning of the verb apistoumen, not “unbelieving,” as KJV, et al.), this does not in any way affect God’s own faithfulness to his people. This can mean either that God will override our infidelity with his grace (as most commentators) or that his overall faithfulness to his gracious gift of eschatological salvation for his people is not negated by the faithlessness of some. This latter seems more in keeping with Paul and the immediate context. Some have proved faithless, but God’s saving faithfulness has not been diminished thereby. So Timothy and the people should continue to endure that they might also reign with him. Thus all four lines cohere as an exposition of “the salvation that comes through Christ Jesus and brings eternal glory” (v. 10). The final coda simply explains why the final apodosis stands as it does: because he cannot disown himself. To do so would mean that God had ceased to be. Hence eschatological salvation is for Paul ultimately rooted in the character of God.

Fee, Gordon D.. 1 and 2 Timothy, Titus (Understanding the Bible Commentary Series) (Kindle Locations 5339-5350). Baker Publishing Group. Kindle Edition.