14:22-23 What is the actual meaning of what Paul says here?

What Paul in effect is saying here is that whatever we believe about the things discussed in Ch 14 up to this point should be kept between ourselves and God. It is not teaching as some claim that faith is private (CP V13-21).

Paul is simply appealing to strong Christians not to force their faith level upon weaker Christians. Strong Christians can adapt their faith to accommodate the weak, but the weak cannot adapt their faith to accommodate the strong without harming their faith.

V23 bears this out. It teaches that those weak in the faith who have doubts about what they eat are condemned if they eat because their eating is not from faith. Whatever is not of faith refers to thoughts and actions that our conscience condemns.

Any thought or action that does not originate and proceed from faith is sin. Those weak in the faith should never act contrary to their conscience, because to violate their conscience is not acting in faith, but is sin (CP V23 with Tit 1:15 and 1Jn 3:16-21).

One must know beyond all doubt that what he participates in is perfectly in accord with God's word before he does it. If he does not have this conviction and goes ahead and does it, it is sin. The statement that whatever is not of faith is sin is controlled by the issues of eating, drinking, and the observance of days here.

Saving faith versus unbelief in the context of comparing unbelievers with believers is not in view. However, in the broad sense, whatever is not of saving faith is also sin (CP Ro 1:18-19 with He 11:6).