20 Do not, for the sake of food, destroy the work of God. Everything is indeed clean, but it is wrong for anyone to make another stumble by what he eats. 21 It is good not to eat meat or drink wine or do anything that causes your brother to stumble.  22 The faith that you have, keep between yourself and God. Blessed is the one who has no reason to pass judgment on himself for what he approves. 23 But whoever has doubts is condemned if he eats, because the eating is not from faith. For whatever does not proceed from faith is sin.  (Romans 14:20-23 (ESV)
Whatever does not proceed from faith is sin. The basic gist here is this, if you believe it is sinful for you to do something then it is sinful for you to do it. It may not be sinful in and of itself. But that you do it believing it is a sin, then you sin. This isn’t to say that it will always be a sin for you to do it either. Perhaps a weak brother gets stronger and what bothered his conscience before no longer does. I used to think gambling was sinful in and of itself. I now regard it the same way I do alcohol, it is something that can be abused, but isn’t necessarily. Some say it is stealing, and it definitely is if a person is cheating at it. But I hardly think a poker game with a group of friends is stealing. Everyone knows the rules, and it can be just a good time. I also find that few other things in life have been able to clear my mind as forty dollars on a black jack table causing me to concentrate on the cards in front of me. It’s a weird form of stress relief. Escapism to be sure. But sometimes that’s what you need for a minute so you can come back and focus on the problems with renewed vigor.
But more than this, what doesn’t proceed from love does not proceed from faith and therefore is sin. If my brother has scruples concerning food and or drink and I purposefully eat and drink in front of him in such a manner as to try to get his goat, then it is sin. That said, there may be a place for that too in a situation. Luther speaks of this in “On the Freedom of a Christian”. There are times when Pharisees make a big deal about what another eats, or drinks. They twist scripture in order to torture his conscience. In the Pieitist revivals that racked the Lutheran world of the 19th century this was done with coffee, tobacco, and alcohol too. In those cases there were others who would share a cup of coffee, smoke and even drink with the weaker brother who was made to think his faith was dependent on giving these things up. Today, I have even seen this applied to tattoos. I keep thinking maybe I should get a tattoo. Just haven’t found the right one, or the money.