When someone says, “The Bible cannot be trusted, it has been corrupted”, they have in mind what they consider a higher standard and are about to recommend something else that can be trusted, that has not been corrupted. When a Roman Catholic says this, they are about to recommend their “Pope” as infallible and more worthy of trust than the Bible. When a Muslim says this, they are about to recommend the Quran.
“The doctrine of infallibility relies on one of the cornerstones of Catholic dogma, that of papal supremacy, whereby the authority of the pope is the ruling agent as to what are accepted as formal beliefs in the Catholic Church.”
https://en.m.wikipedia.org/wiki/Papal_infallibility