Imagine a huge group in the Church had a council in the year X, refused to invoke the Holy Ghost, and ended up deciding to completely change everything about the Catholic Church. From then on, they never reference anything before the year X, except maybe to deride and criticize it. All popes after that council act as if the church they belong to literally began in year X. The popes after this council only refer to each other -- never to the popes before the council.
What would you call that new church? A heretical sect? A new church in schism with the Catholic Church?
Obviously, this is not exactly hypothetical. It actually happened. The year X was 1965, and the new sect is the "conciliar" church, or the Church of the (Vatican 2) Council.