Romans 14

But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin. v.23

Read chapter 14
Eating in faith?  What does that even mean?  When we eat are we only to eat if we are told by God to?  If we forget to pray before we eat does that mean were sinning?  Sounds foolish right.  Well if you don't read the context of this whole chapter but take this out of context it is foolish.  This chapter is talking about people's conscious of eating foods that were once unclean that now God has declared clean.  Paul was saying that even if they knew that eating a certain food is fine but someone else is grieved by it in their consciousness with God for they don't know God's word about the matter, they ought not to eat whatever food to not cause their brother to stumble for it is sin of their conscious before God (see verses 14-16).   No food today in our culture seems that it would cause someone to stumble, but what about alcohol for example?  Our world has many alcoholics and it's common practice if someone is a newly recovering alcoholic in our midst that there's no temptation of alcohol present at a gathering to help the person not be made to stumble.  Ought we to then avoid things that would cause someone to stumble, even if our conscious is clear before God?
What God has declared about what is clean and unclean to eat is taking His word and then acting on it.  So God declaring all food clean, an Israelite no longer needs to avoid eating pork for example.  So saying whatever is not from faith is sin is not taking God at His word.  And to not take God's "word for it" is to sin- whether it's about eating, sex, love, pride, all matters of life. 
What might you need to avoid to help a friend or family's struggle with a stumbling block in their life, even though it's not in yours?