But he who doubts is condemned if he eats, because his eating is not from faith; and whatever is not from faith is sin (Romans 14:23).
Does this mean that if I believe what I do is wrong, even if it is not inherently a sin by something in God's Word, it becomes a sin for me? In short, yes.
Remember, that this verse is in the larger context of Paul's discussion of the relationship between the Jewish and Gentile believers. In what sense would the Jewish believer eating, say a pig, be sin? If the Jewish believer still thinks of himself as under the Mosaic Law, then eating the pig would be a violation of the Law (i.e. a pig is an "unclean" animal), and it would be sin for him. The same logic is applied to the believer coming out of an idolatrous backgound. If they eat food sacrificed to idols, thinking that the idol is real, then to them it would be sin (1 Corinthians 8:7, 10-11).
In other words, what a person believes governs whether something is sin for them or not.
Now, the converse of this would not be true; that is, if I believe something is right, but the Scriptures call it sin, it does not, therefore, become right. It is still sin. In other words, conscience itself is not always sufficient to judge what is right; nevertheless, it can condemn us by removing us from the realm of faith. Therefore, we are encouraged to always live by faith.