Cardinal Robert Bellarmine (1542–1621) was a figure not to be taken lightly. He was Pope Clement
VIII’s
personal theologian and one of the most able figures in the
Counter-Reformation movement within sixteenth-century Roman Catholicism.
On one occasion, he wrote: “The greatest of all Protestant heresies is assurance.”
By Sinclair Ferguson
http://www.ligonier.org/blog/what-greatest-all-protestant-heresies/