The original Portrait of Pope Innocent X, by Diego Velázquez in 1650:

"Velázquez was commissioned by Innocent X to paint the portrait as from life, the pope’s motive being to increase his prestige. However, Velázquez did not flatter his sitter and the painting is noted for its realism, being an unflinching portrait of a highly intelligent and shrewd, but aging man.

Bacon never painted from life, preferring to use a variety of visual source material, including photographs both found (including in movie stills, medical text books and 19th century journals) and commissioned. Equally, Bacon rarely worked from commission, and could portray the pope in an even less flattering light; according to art critic Arim Zweite, “in a sinister manner, in cavernous dungeons, afflicted by an emotional outburst and devoid of any authority”.[8]

Although Bacon avoided seeing the original, the painting remains the single greatest influence on him; its presence can be seen in many of his best works from the late 1940s to the early 1960s. In Bacon’s version of the 17th century masterpiece, the Pope is shown screaming yet his voice is “silenced” by the enclosing drapes and dark rich colors. The dark colors of the background lend a grotesque and nightmarish tone to the painting.[9] Although a noted bon vivant, Bacon closely guarded his private life, working habits, and thought processes. He produced some 50 paintings of popes, but destroyed a great many that he was unhappy with"

https://en.wikipedia.org/wiki/Study_after_Velázquez's_Portrait_of_Pope_Innocent_X