This is a very broad topic but I think that it centers around unity with the Magisterium. From that point it follows that faith is directly related to the obligation of obedience. Once a person ceases to be obedient to the teaching authority of the Church they find themselves in either formal or material heresy and as such no longer united to the Church. Adherence to the faith is necessarily an external and internal assent to a source that is outside of ones self. If the source is internal then it ceases to hold to an objective unity.