In other words, “if you think it's sinful, it is, even if it's not.”? Pity the poor scrupulous?!
In this particular case, Paul is saying that meat consecrated to idols is fine, but if someone thinks it isn't fine and does it because he sees you partaking in it, then you've contributed to that fellow's downfall.
The Bible commentaries have varied things to say:
the weak Christian will be undermined: he will be encouraged to act against his (erroneous) conscience, and all acts against conscience are sinful…. [Those who know the meat is okay] have overlooked Christ’s teaching about stumbling-blocks (skandala): that an act lawful in itself may become even a mortal sin if it is foreseen that it will place difficulties or temptations in the way of a weaker Christian.
Consciousness (syneidēsis, vv. 10b, 12) arises from knowledge (syn-eidenai). The term “consciousness” first appeared in the papyri as of 59 c.e. Paul probably took the term over from its use in popular philosophy. As used by Paul it retains its traditional meaning of self-awareness. There is no need to see in Paul’s use of the term the modern notion of moral conscience.
Those with a weak consciousness... Their old habits had left a residue on their self-awareness such that it was not governed by their present Christian beliefs. Their self-awareness would be defiled were they to eat food they considered to have been offered to idols.
Those who are weak would be led to idolatry because of the knowledgeable person’s indiscriminate eating in temple precincts. They would eat food offered to idols as if it were truly dedicated to one or another idol
Their salvation (cf. 8:6) is lost because they have been led to engage in what they considered to be idol worship.
It is the believer’s responsibility not to trip up weaker persons (Chrysostom) who might think that there is some spiritual power in food offered to idols, a power they might acquire if they eat (Ambrosiaster).So I guess the problem is that some of these people who thought eating meat sacrificed to the gods was sinful, ended up doing so anyway and felt some sort of divine benefit from it. Maybe it's sort of like the guy who tells another guy that drinking a pint is not sinful, but for the other guy, call him John, it always leads to sin in the form of, say, cleptomania and he derives the "benefit" of theft. But that's not the same as John thinking drinking itself is sinful and thus is going against his conscience which is, thereby, sinful. Maybe the act of eating it and going against his conscience was not the sinful part so much as feeling that the fake gods were in fact real?
And also “conscience” as self-awareness is interesting given how we associate it with the modern moral conscience. Are these concepts so different?