As I already explained to you in post #21663 - The word "if" here is not ean, an unfulfilled, hypothetical condition used with the subjunctive mode, presenting the possibility of a future realization, but ei with the indicative, having here the idea of "assuming that you continue in the faith." -
Steadfast (firm) (1476) hedraios - Sermon Index
That is, 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 in Greek (using the Greek particle ei and the indicative mood of the verb epimenō) indicates that Paul fully expects that the Colossian believers will continue in the faith; no doubt is expressed, yet what about "nominal" Christians mixed in with the group whose shallow, temporary belief withers away?
It's only natural that Paul would speak this way, for he is addressing groups of people who profess to be Christians, without being able to know the actual state of every person's heart. How can Paul avoid giving them false assurance here that they will be eternally 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 they are genuine believers. But those who do not continue show that their shallow faith was not grounded in the gospel to begin with.