what happens if you find out the pope used to be a heretic

[SaintSebastian]

I found another example from the late 6th, early 7th century. I actually found it in  a 15th century Concilliarist apologetic where it is used to sum up the opponent's position. This Concilliarist then tries to say (without any examples or solid justification, IMO) that this rule only applies to good Popes, not wicked one (this Concilliarist author would later abandon this position and he even became Pope himself: Pius II):

St. Isadore of Seville: "The authority of gathering synods was given to the power of the Apostolic See alone, nor do we read that any synod was valid which was not gathered or supported by its authority." (CIC 1:51 D. 17 c. 2)

[glgas]

Only pope (not even a council) can declare that a pope is heretic.

This is incorrect.  The 6th Ecumenical council declared Pope Honorius to be a heretic.  The Roman legates in attendance did not oppose the condemnation.

And with their approval completed the declaration.

[Benno]

Just replying to the original thread topic -
a) you learn about it
b) you find out if it's true or not
c) you have faith in the office of the pope
d) you say your prayers, do good for others and hope for the best