But the one who doubts, if he eats, condemns himself because it is not out from faith. Indeed, all that is not done out from faith is sin (Romans 14:23).
Doubting involves a thorough judging for the purpose of justifying actions that are known to be wrong. When we do something that we know is wrong—and therefore not out from faith, for we are not taking God at His word in our actions—we sin.
Sin is an act of lawlessness (John 3:4); it is not within the mind. Therefore, let us be cautious to maintain our faith in our conduct, not imposing it upon others (Romans 14:22), and not violating our conscience due to peer pressure.