Since conscience is your own relationship to the objective moral truth, you are obliged to follow your conscience. In other words, what one determines to be good and right based on the use of one's reason is like a law that must be obeyed (Gaudium et spes, 16). It is the closest approximation one has to the truly good. It is the way that one participates in God's goodness. It is here that one is 'alone with God, whose voice echoes' in one's depths (GS 16). So, to not obey your conscience, to choose not to do what you know to be the morally right thing to do or to blindly obey others is to abdicate personal responsibility for moral decisions. to act in this way is a sin because it is tantamount to idolatry. If conscience is where we relate most closely at a personal level with the objective truth of God and perfect goodness, then to do other than what we believe to be right and true in our conscience is to do other that what we believe to be God's will, to do other than what we believe to be perfect goodness. That is the essence of what we mean when we talk about sin.