There has to be a canon of truth against which the teachings are based.
What is it?
Such easy questions. The constant teaching of the Church is based upon the revelation of Jesus Christ as handed down to the Apostles and preserved both in Sacred Scripture and also in Sacred Tradition. This is in contrast to the novelties of the so-called Reformers who crafted teachings that were unknown to Christianity for 1500 years.
Further, could not the current pope claim his teaching is catholic tradition?
What do you measure it by?
Further, who gets to measure it and decide if it's heretical or not?
Which of the following 21 church councils are THE Truth for Catholics?
New Advent - The 21 Ecumenical Councils
http://www.newadvent.org/library/almanac_14388a.htm
Which one represents the OTC Catholic church?