Piper's arguments essentially refer to some of the most fundamental differences between Roman Catholics and Protestants, at a time when the former face a serious crisis, with calls for Pope Francis to stand down.

The Catholic Church has been hit by revelations of wide-spread sex abuse of children at the hands of priests and subsequent clergy cover-up that went on for decades, for which Francis has been apologizing.

Notable figures, such as Archbishop Carlo Maria Viganò, who previously served as the Vatican's ambassador to the United States, have recently accused Francis of being part of such cover-ups himself, however, and have called on him to resign.

In a striking 11-page letter, Viganò accused Francis of covering for disgraced former U.S. Cardinal Theodore McCarrick after the latter took sexual advantage of young seminarians.

"Pope Francis must be the first to set a good example for cardinals and bishops who covered up McCarrick's abuses and resign along with all of them," Viganò demanded in the letter.

When asked by journalists to respond to the accusations, Francis said on Sunday:

"I will say sincerely that I must say this, to you and all of you who are interested: Read the document carefully and judge it for yourselves."

"I will not say one word on this. I think the statement speaks for itself," the pontiff added.

"You have sufficient journalistic capacity to draw conclusions. When a little time has passed and you have the conclusions, perhaps I will talk."

Piper, who is the former pastor of Bethlehem Baptist Church, has in the past argued that the "crux" of both the Reformation and the Gospel is the concept that people are justified and ultimately saved by the grace of God, rather than by the works of man.

"Especially as it pertains to final salvation, so many of us live in a fog of confusion," Piper wrote in 2017.

He also pointed to the Reformation's five "solas," namely sola Scriptura (Scripture alone), sola fide (faith alone), sola gratia (grace alone), solus Christus (Christ alone), and soli Deo gloria (to the glory of God alone).

A Pew survey from earlier in 2017 found that 52 percent of U.S. Protestants believe that both good deeds and faith are needed for salvation, despite the belief being linked closer to Catholics. Still, 46 percent of Protestants agreed with the belief of faith alone.