In Col 1:23, we read:
If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister.
The use of "if" is part of the underlying Greek text, using ει, which, according to BDAG is a "marker of a condition, existing in fact or hypothetical." That Paul is commanding true believers/Christians to continue in their faith, as they could fall away and lose their salvation, is reflected in many modern translations of the Bible, such as the NRSV ("perhaps") and even Protestant translations ("provided that" [NASB]). Indeed, BDAG actually references Col 1:23 thusly:
εἴ γε if indeed, inasmuch as (Kühner-G. II 177c) Eph 3:2; 4:21; Col 1:23. τοσαῦτα ἐπάθετε εἰκῇ; εἴ γε καὶ εἰκῇ have you experienced so many things in vain? If it really was in vain Gal 3:4. εἴ γε καὶ ἐκδυσάμενοι οὐ γυμνοὶ εὑρεθησόμεθα assuming, of course, that having put it off we shall not be found naked 2 Cor 5:3. [εἴ γ]ε οὕτως ὡς [ἔστιν καὶ παρελάβετε τὸν λόγον] AcPl BMM recto, 31f (restoration based on duplicate Ox 1602 verso, 37f and AcPl Ha 8, 24f, which has a slightly difft. text after εἴ γε [s. also the text of Ghent 62, 17 in HSanders, HTR 31, ’38, 79, n. 2]). S. γέ bα.
Let us see how Protestant commentaries approach this verse:
E.K. Simpson and F.F. Bruce
This, then, is the prospect which lies before them, provided that they remain firmly founded and established in their faith. If the Bible teaches the final perseverance of the saints, it also teaches that the saints are those who finally persevere—in Christ. Continuance is the test of reality. The apostle’s language may suggest that his readers’ first enthusiasm was being dimmed, that they were in danger of shifting from the fixed ground of the Christian hope. And indeed, to hold fast to hope is throughout the NT an indispensable condition of attaining the goal of full salvation to be revealed at the parousia of Christ. Hope forms an essential part of the gospel—that gospel which (as Paul has already emphasized) is spreading and bearing fruit in all the world, having been proclaimed (as he puts it here, perhaps indulging in a prophetic prolepsis) “in the whole creation under heaven.” The catholicity of the gospel is a token of its divine origin and power. And of this gospel—wonder of wonders!—“I Paul have become a minister.” He sees his personal ministry as closely bound up with God’s gracious plan for the world. (E.K. Simpson and F.F. Bruce, Commentary on the Epistles to the Ephesians and Colossians: The English Text with Introduction, Exposition and Notes [NICNT; Grand Rapids, Mich.: Eerdmans, 1957], 213)
James D.G. Dunn
The confidence in the effectiveness of the divine provision made for those estranged from God by heir evil and for the blameworthy by Christ’s death is qualified by a matching emphasis on human responsibility. Such emphasis on the need for persistence in Christian belief and conduct is a regular feature in Paul (e.g., Rom. 8:13, 17; 11:22; 1 Cor. 9:27; 10:11-12; Gal. 5:4). And should not be ignored. Ει γε may denote confidence more than doubt (cf. its use in 2 Cor. 5:3; Eph. 3:2; 4:21), but final acceptance is nevertheless dependent on remaining in the faith. The parenetic and pastoral point is that however such persistence must be and is enabled by God through his Spirit (1:11), there must be such persistence (cf. O’Brien, Colossians, Philemon 69) . . . Paul always insisted that the ongoing “walk” (1:10) of the Christian should be in direct continuity with and continuingly expressive of the faith by which the Christian first entered upon that walk (the main thrust of Galatians; so, e.g., Gal. 3:2-3; 5:4-6; Rom. 14:23). It is probably that faith by which the Colossians first received the gospel which is referred to here: without that same basic conviction and openness to the grace of God they would be unable to persistent (Aletti, Épitre aux Colossiens 126). On the other hand, the definite article could denote an early example of the objectification of faith (“the faith”; Houlden 175; O’Brien Colossians, Philemon 69) which begins to characterize post-Pauline usage (1 Tim. 3:9; 4:1, 6; 5:8; 6:10, 21; etc.), though it could equally denote “your faith.” Pokorny 93 thinks both meanings are involved here.
The point is reinforced by a sequence of strengthening images. “Established” (εθεμελιωμένοι) uses the image of a foundation” (θεμελιος). The verb occurs only here and in Eph. 3:17 in the Pauline corpus, but Paul liked to think of himself as a master builder laying a foundation of the gospel or of faith in Christ (Rom. 15:20; 1 Cor. 3:10-12). The image of Christ as the “foundation” on which Christians are established (1 Cor. 3:11) was presumably drawn from Isa. 28:16 (c. particularly Rom. 9:33; Eph. 2:20; 1 Pet. 2:6). The passive here could imply that Paul or Epaphras as the builder; as elsewhere, the “provide that” takes its force from the gospel in which the Colossians first believed (1 Cor. 15:2; Gal. 5:4-5). Εδραιος (“firm, steadfast”; elsewhere in the New Testament only in 1 Cor. 7:37 and 15:58; but also in Ignatius, Ephesians 10:2; Polycarp 3:1) comes from εδρα or εδρη, meaning a “place where one sits”: the addresses are to remain as firmly seated on the gospel as a god in his temple or a skillful rider on a spirited horse. The third image is simply that of movement, shifting from one place to another (μεταχινουμαι; only here in the New Testament but echoing the imagery in 1 Cor. 15:58; cf. Deut. 19:14; not quite “drift away,” as in JB/NJB). His they must avoid, remaining firmly attached to “the hope of the gospel,” a neat phrase summarizing the earlier emphasis (1:4-5), with “hope” again prominent as characterizing the “gospel” (see on 1:5). (James D.G. Dunn, The Epistles to the Colossians and to Philemon [New International Greek Testament Commentary; Grand Rapids, Mich.: Eerdmans, 1996], 110, 111)
It should be obvious that Paul is warning true, not mere superficial, Christians that they must continue; if not, they can and will lose their salvation; previously in Romans ch. 4 he used the example of King David who lost his justification and had to regain such (see King David Refutes Reformed Soteriology).
That Paul is not teaching anything that can be considered historic, Reformed theology can be seen also in the very next verse, where, instead of teaching a penal substitutionary model of atonement, he instead teaches a participatory model:
Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's take, which is the church.
The NASB offers the following translation:
Now I rejoice in my sufferings for your sake, and in my flesh I do my share on behalf of His body, which is the church, in filling up what is lacking in Christ's afflictions.
Commenting on this verse, Larry Hurtado wrote the following:
Paul’s sufferings are pictured strikingly as allowing him to complete in his own flesh what is lacking in Christ’s afflictions for the sake of the church. In the preceding verses (1:15-23), Christ’s preeminence over all creation, and the universal scope of this redemptive work, are celebrated; and this reconciliation is specifically described as accomplished through Christ’s own fleshy body in his death (v. 22). As commentators rightly note, behind the reference to completing what is “lacking” in Christ’s afflictions probably lies the idea of an eschatological quota of sufferings (messianic “woes”) that must be completed so that the final consummation of God’s redemptive plan might take place. That is, the idea here is not that Paul’s sufferings compensate for an insufficiency in Christ’s sufferings, or that Paul’s sufferings contribute to the redemption of the church. Instead, the things that Paul suffered are pictured here as affording him the chance, on behalf of the church, to make a special contribution to the eschatological measure/quota of eschatological sufferings. Yet it should be clear also that the traditional apocalyptic idea of eschatological sufferings has been reshaped here by the crucifixion of Jesus, so that Paul and other believers can undergo their sufferings as service to the crucified Jesus. We may also note Philippians 1:29, where Paul reminds his readers that it has been given them (by God) both to believe in Christ and also “to suffer for his sake,” and Philippians 3:10, where Paul expresses his deep and continuing aspiration “to know him [Christ] and the power of his resurrection and the participating/sharing of his sufferings [κοινωνίαν [παθημάτων αὐτοῦ], being conformed [συμμορφιζόμενος] to his death.” (Larry Hurtado, "Jesus' Death as Paradigmatic In the New Testament" in Ancient Jewish Monotheism and Early Christian Devotion: The Context and Character of Christological Faith [Waco, Tax.: Baylor University Press, 2017], 351-71, here, pp. 365-66)
Hurtado is correct in stating that Paul is not speaking of the (eschatological) salvation of the Church (esp. as the individual is in view in v. 23!), but Paul is clearly teaching a form of participatory atonement. Morna Hooker noted the following about this often puzzling passage:
Colossians 1:24 provides an interesting example of the way in which commentators have allowed their theological convictions to influence their interpretation of the text. The belief that Christ’s death is decisive and once-for-all has led some of them to shy away from the straight-forward meaning of the words. Another example of this can be seen in the refusal to allow that Paul ever speaks of imitating Christ. Colossians 1:24 reflects the conviction that we have found elsewhere in Paul’s writings, that it is necessary for the Christian to share in the sufferings of Christ and that this participation in suffering can be of benefit to other members of the Christian community. This necessity is not based on the idea that there is a set quota of messianic sufferings that need to be completed. Rather it arises from the representative character of Christ’s death. If Christ died for all, this means not only that all have died, but that they must continue to work out the meaning of dying with Christ. The acceptance of Jesus as Messiah means a willingness to share his experiences. In this sense, at least, the sufferings of Christ are no substitute for ours, but a pattern to which we need to be conformed.
The tendency to stress the belief that Christ’s death was a substitute for ours to the exclusion of the Pauline conviction that Christians must participate in the suffering of Christ is perhaps a very early one. The Corinthians, e.g., seem to have been unable to grasp the idea that there was any place for suffering and humiliation. In their calling: for them, resurrection with Christ was a past event, and this meant that they shared already in his glory, fullness, and riches (cf. 1 Corinthians 4:8). Christ had suffered—and they experienced the resulting glory. He had become for them the substitute for humiliation and death. They failed to see the necessity to share his sufferings. (Morna D. Hooker, “Interchange and Suffering,” in Suffering and Martyrdom in the New Testament, eds. William Horbury and Brian McNeil [Cambridge: Cambridge University Press, 1981], pp. 70-83, here, p.82, emphasis added).
Col 1:23 (and v. 24) is another "un-Protestant" passage of the Bible, just as many others, such as 1 Cor 3:15, are.