Romans 14:23
"And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin."
Cross References (ranked by relevance)
But without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.
Unto the pure all things are pure: but unto them that are defiled and unbelieving is nothing pure; but even their mind and conscience is defiled.
Howbeit there is not in every man that knowledge: for some with conscience of the idol unto this hour eat it as a thing offered unto an idol; and their conscience being weak is defiled.
One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind.
Whosoever therefore resisteth the power, resisteth the ordinance of God: and they that resist shall receive to themselves damnation.