Yesterday we learned not to judge or be jealous of others because of their differing convictions concerning non-essential matters.  Today, as Romans 14 continues, we are told to take the matter to another level.  
We are given two major exhortations: 1.) When we know there are other believers with us who hold a “stricter” standard than we do and that it would be offensive to them for us to participate, we should refrain.  2.) Whenever we violate our conscience, even if what we are doing is perfectly fine, we are sinning (“whatever is not of faith is sin”).
Both of these truths have to do with our motivations.  Being willing to put aside our normal practices in order to not lay a stumbling block before our brothers and sisters should be done out of a motivation to love.  Also, if we thought it was sinful to where blue shirts on Tuesdays and we decided to wear a blue shirt today anyways…that would be rebellion.  If I truly believed that it was sinful, and I decided to go ahead and wear my blue shirt, I would be communicating to God that I have disregarded His will (even though I have a very poor understanding of it).  The truth is, we all may have some convictions that just are not in the Bible.  But, if I, still believing my conviction to be true and right, violate that standard, I have rebelled.  It is a matter of the heart more than a matter of the law.