>>100169796started that way, but Martin Luther also changed the theology
solis fidel (Saved through faith alone)
The belief that if you believe Jesus is your savior you're 100% going to heaven no matter what. Idk if that's actually what Luther believed, but it's a common belief among modern protestants.
solis scriptura (Scripture alone)
The Bible is the only source of truth (not really sure what the correct word would be). There's some things the Catholic church believes pretty much equally with the Bible, even though it's not explicitly in the Bible (Mary was a virgin for life, Mary's assumption into heaven, etc)
solis gratia (Grace alone)
You're saved by the grace of God alone, not through any of your own efforts. This is shared with the Catholic church, but there's an autistic argument about works (really this is more about solis fide). Protestants believe in solis fidel whereas Catholics believe that if you believe and love God you'll inevitably do works, so technically solis fidel isn't true.
As for corruption, the Catholic church doesn't claim it was or is free of corruption, their belief is that their theology is correct and corruption of memebers of the church doesn't detract from the theology or the sacraments.
ie if your local priest is a pedophile, the communion he consecrates is exactly the communion consecrated by an upright priest. His absolution in confession is just as valid, etc. Basically it's like it a cop is corrupt and arrests you for murder. The cop is acting with legitimate authority, his other corrupt actions don't invalidate his actions when doing something correct/just.
Oh yeah and communion is a big one. Protestants generally believe communion is just symbolic whereas Catholics believe it's literally the body and blood of Christ. Also praying for the intercession of Saints, which leads to some Southern Baptist types thinking Catholics are blood drinking pagans.