The Grace Reading
Everything in these verses is settled until the last one, and then a single word seems to unlock the door again. If. Read quickly, it sounds like the reconciliation Paul just described is being held hostage by your staying power.
Start with what he says has already happened. You were alienated, hostile in your mind, occupied with wicked works. Now He has reconciled you, in the body of His flesh, through death. And the stated purpose is not an outcome He is hoping to reach: “to present you holy, and blameless, and above reproach in His sight.” Not improved. Not provisional. Blameless, in His sight, on the strength of a death that already occurred.
So what is the condition guarding? Look at the words Paul chose. Continue in the faith. Grounded and steadfast. “Not moved away from the hope of the gospel which you heard.” That is the language of a building on a foundation and a message you were handed. It is not a gauge measuring how warmly you have felt about Jesus lately.
Which is exactly the danger in Colossae. The teachers there were not telling anyone to abandon Christ. They were telling them Christ was not enough. Angels to negotiate with, festivals to observe, food to avoid, regulations with the ring of wisdom: “Do not touch, do not taste, do not handle” (2:21). Paul's answer runs through the entire letter. “You are complete in Him” (2:10). Do not let anyone move you off the ground you are standing on and onto an upgrade that was never real.
So the “if” is a question about your foundation, not about the Father's grip. Paul settles the grip two chapters later: “For you died, and your life is hidden with Christ in God” (3:3). Ask the old question, who shall stand in His presence with clean hands and a pure heart, and the New Covenant answer is that you shall, because He made Him who knew no sin to be sin for you, that you might become the righteousness of God in Him (2 Corinthians 5:21).
Stay on the gospel you first heard. There is nothing to add to it, and the One who reconciled you is the One presenting you.
The Common Misreading
Read as a probation clause, verse 23 turns the paragraph inside out: holy, blameless, above reproach, pending review. Believers begin scanning themselves for enough continuance to keep the verdict, which is precisely the self-inspection the letter was written to end. The result is a Christian who can recite the reconciliation and never once rest in it.
Paul is not dangling your standing over the edge. He is naming the one real threat in that city, that someone would move these people off the gospel and onto a system of add-ons, and he is telling them to stay planted. The foundation holds you. You were never the one holding it.