In regards regards to Colossians 1:23 - ..if indeed you continue in the faith, grounded and steadfast, and are not moved away from the hope of the gospel which you heard, which was preached to every creature under heaven, of which I, Paul, became a minister.
Continuance would show that the person's faith is firmly established in the hope of the gospel and they really have been reconciled. The form of this phrase indicates that Paul fully expects that the Colossian believers will continue in the faith; no doubt is expressed, yet what about "nominal" Christians whose profession of faith does not continue?
It's only natural that Paul would speak this way, because he is addressing groups of people who "profess" to be Christians, without being able to infallibly know the actual state of every person's heart. How can Paul avoid giving them false assurance of salvation here that they will be saved when in fact they may not? Paul knows that faith which is firmly grounded and established in the gospel from the start will continue. Those who continue in the faith show thereby that their faith is in fact grounded in the gospel and they are genuine believers. But those who do not continue show that their spurious faith was not grounded in the gospel to begin with.
Just as we see in 1 Corinthians 15:1,2 - Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast (demonstrative evidence of faith being firmly rooted and established) that word which I preached to you--unless you believed in vain. To believe in vain is to believe without cause or without effect, to no purpose. If as some are saying in Corinth, there is no resurrection, then faith is vain and worthless (vs. 14). The people who fail to hold fast to the word (the gospel) that Paul preached in 1 Corinthians 15:1-4, demonstrated that they "believed in vain" (did not truly believe).