Paul then writes, more controversially, that enjoying an unblemished state through Christ's shed blood (i.e. his death) is only possible εἴ γε ἐπιμένετε τῇ πίστει τεθεμελιωμένοι καὶ ἑδραῖοι καὶ μὴ μετακινούμενοι ἀπὸ τῆς ἐλπίδος τοῦ εὐαγγελίου οὗ ἠκούσατε (Colossians 1:23).
Prima facie this passage appears to teach us that "eternal security" could be a fictive theological construct that is not to be found in Scripture. Paul's inspired words seem to imply that a Christian must remain in the faith while adhering to the hope of the Gospel that was preached ἐν πάσῃ κτίσει τῇ ὑπὸ τὸν οὐρανόν so that he/she might be saved in a definitive sense. This verse thus appears to militate against any notion of eternal security; hence, at this point, it would be beneficial to see what some first-rate commentaries have observed about the conditional structure of this textual unit.
Peter O'Brien observes that Colossians 1:23 is a lengthy conditional sentence containing both positive and negative elements (O'Brien, Colossians, Philemon 69). He maintains that εἴ γε "does not express doubt" (Ibid); rather, he believes that the apostle is really saying, in so many words, "At any rate if you stand firm in the faith--and I am sure you will . . . " He takes this position in view of Colossians 2:5 (Ibid).
O'Brien however goes on to write that "If it is true that the saints will persevere to the end, then it is equally true that the saints must persevere to the end' (Ibid). He then attempts to show how Colossians constitutes an exhortatory message, which strengthens the "saints" to avoid lapsing into a state of smug complacency. O'Brien therefore does not believe that Colossians 1:22-23 fazes the doctrine of eternal security at all. When interviewed about this subject, he provided this response:
"The warnings of Hebrews [6:4-8; 10:26-39, etc.] have presented many challenges to believers throughout Christian history. And the misapplication of them has caused pastoral problems for Christians of all traditions, including the Reformed. These warnings have troubled earnest Christians by raising doubts about their assurance of salvation, an assurance that is so clearly affirmed, for example, in Romans 5:1-11 and Romans 8:18-39, and in Jesus's promises for his disciples in John 6:39-40, 44 and John 10:25-30."
James Dunn, on the other hand, provides this commentary:
"εἴ 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 the Spirit (1:11), there must be such persistence (cf. O'Brien, Colossians, Philemon 69)' (Dunn, Colossians, Philemon 110).
Dunn (in ftn 8, page 110) then quotes R.W. Wall as follows:
"Paul does not teach a 'once saved, always saved' kind of religion; nor does he understand faith as a 'once for all decision for Christ.'"
You can find Wall's remarks in full by consulting his commentary, Colossians and Philemon (The IVP NT Commentary; Downers Grove: InterVarsity, 1993), page 81.
I believe that Wall's understanding of Colossians 1:23 is closer to what Paul wanted to say with his use of εἴ γε, KTL.