I get the impression that some people aren't understanding the context of this verse.
The NLT version conveys the meaning of this chapter in modern English:
1 Accept other believers who are weak in faith, and don’t argue with them about what they think is right or wrong.
2 For instance, one person believes it’s all right to eat anything.
But another believer with a sensitive conscience will eat only vegetables.
…
15And if another believer is distressed by what you eat, you are not acting in love if you eat it.
Don’t let your eating ruin someone for whom Christ died.
21It is better not to eat meat or drink wine or do anything else if it might cause another believer to stumble.
22You may believe there’s nothing wrong with what you are doing, but keep it between yourself and God.
Blessed are those who don’t feel guilty for doing something they have decided is right.
23But if you have doubts about whether or not you should eat something, you are sinning if you go ahead and do it.
For you are not following your convictions.
If you do anything you believe is not right, you are sinning.
Many of the Greek proselytes have spent their whole lives as vegetarians, believing that eating the flesh of animals is a sin.
Most of them are also ascetics, believing that physical pleasure is a sin.
The Bible teaches that the flesh of clean animals is not only allowed as food, it is to be enjoyed as food, and the Jewish Christians take this lifestyle for granted.
Some of them think the Greeks that are learning about Christianity should immediately accept being freed from the pagan restrictions of their past and should immediately start eating animal flesh, drinking wine, and generally enjoying life.
But for these proselytes, this is distressing.
Even those that have intellectually accepted God's truth, can't instantly put it into practice.
(Consider if you learned that eating live cockroaches dipped in worm purée was an enjoyable experience.
How comfortable would you feel watching others eat it?
How would you feel when others encourage you to eat it?
How would you feel when others put you down for not eating it?)
Paul's point here is that by belittling the proselytes that are vegetarians, the converted Christians are hurting the cause.
These proselytes are going to feel like they would be doing wrong to eat that food.
And as a result of these superficial aspects of conversion, many of the vegetarians may end up leaving the church before they ever have a chance to learn its fundamental truths.
For them to deliberately go against this feeling of doing wrong would be, for them, the equivalent of sinning (breaking the law).
This verse is obviously not defining sin as "sin is whatever is not of faith" (i.e. doing something that one has doubts about).
The intended meaning is clearly "whatever is not of faith is sin".
x∉F ⇒ x∈Sversusx∈S ⇒ x∉F. – Ray Butterworth Apr 20 '23 at 14:43