Article: Protestants in general reject the veneration and invocation of the Saints.:1174 Protestants typically hold that Mary was the mother of Jesus, but was an ordinary woman devoted to God. Therefore, there is virtually no Marian veneration, Marian feasts, Marian pilgrimages, Marian art, Marian music or Marian spirituality in today's Protestant communities. Within these views, Roman Catholic beliefs and practices are at times rejected, e.g., theologian Karl Barth wrote that "the heresy of the Catholic Church is its Mariology".

Now answer this question: Which theologian described Mariology as "the heresy of the Catholic Church?"
Karl Barth