A
Axion
Guest
On the original question, I know of two people who might possibly fit the bill
- Cardinal Contarini: who formed a movement called the Spirituali in the 1530s, who adapted several Lutheran ideas including justification by Faith. They produced books that presented Lutheran ideas in Catholic ways. Contarini tried to negotiate a compromise agreement at the Conference of Regensburg in 1541, but it was considered that he had given away too much doctrinally to the Lutherans, and the agreement fell through. Contarini remained Catholic, but many of his supporters left the Church.
- I may get jumped on for this, but I would also add, Cornelius Jansen, a 17th Century Dutch catholic who, although he remained staunchly Catholic, adopted many tenets on predestination and the lack of free will that followed those of Calvinism. His teachings were largely condemned as heretical, but he had a large following, particularly in areas where Calvinism was strong like France and Holland, and his movement caused a lot of conflict within the Church.