"The opponents of the pope, including unfortunately even prominent bishops within the church, will stop at nothing to stop him steering his steady course," the Pro Sancta Ecclesia group wrote in an advert in the daily Frankfurter Allgemeine Zeitung.
"All the more reason for Catholics who are true to their beliefs and the Church to rise up against these attempts, stand behind the pope and support him with prayer and work, word and deed," the group added.
There has been a "manipulative media campaign" against Pope Benedict XVI with "excessive blasphemy," the group said, vowing to stand squarely behind the pontiff.
In a separate advert in the same paper, the German Catholic Forum called on all Catholics to "stand unequivocally at the pope's side" and rejected "any political interference in internal church matters."
The pope's decision to lift the excommunication of Richard Williamson - a British-born bishop who denied the nazis used gas chambers to kill Jews - caused outrage in his native Germany.
The row prompted a highly unusual intervention from Chancellor Angela Merkel who called on the Vatican Tuesday to clarify its position. In a climbdown the next day, the Vatican called on Williamson to recant his position but did not reverse its decision to lift his excommunication.
Merkel Thursday praised the Vatican's statement, describing it as an "important and good signal" and a "step forward" in the dispute.
Her uncompromising approach to the controversy won Merkel widespread praise in Germany - where denying the Holocaust is illegal. Bishops, Jewish groups and media editorialists lined up to laud Merkel's comments.