First, Protestants don't even believe that. When the Word says "you are not saved by faith alone" (James 2, I slightly paraphrase) or "This is my body" (Luke 22), you don't believe that.
Second, you don't believe that directly. You believe that because the Church went through the effort of putting the Canonical Scripture together for you. So you believe what the Church tells you, because the Church has told you that. But the distinction "I believe the scripture but I don't believe the rest of the teaching of the Church is foolishness. For one thing, it is nowhere to be found in scripture. Further, it makes no sense: you either believe the witness or you don't, regardless of the method of delivery of the testimony.
most Catholics were not even given a chance to read what the Bible actually said until 500 or so years ago
Anyone could read if he could and wanted to. Most simply could not read, Catholics and Protestants alike. The reason Lutehr was able to pull a fast one over the German peasants was that they were all illiterate.
disobey this mandate/commandment was a mortal sin
It was a sin. It still is a sin. You have to obey what the Church mandates. That part did not change. The mandate itself changed, because the Church saw it fit to change it. The Church has that authority, you know (Mt 18:18).
Catholics don't believe the Word themselves.
When the Word says "you are saved by grace through faith " (Ephesians 2, slight paraphrase) or *Abstain from blood* (Acts 15), *by the works of the Law shall no flesh be justified* (Romans 3) they don't believe that.
Sin is disobeying God's Law, not the mandates of a man-made religion.
And the RCC claiming that it has authority given by God to decide what is and is not sin is blasphemy.
The *church* can't just make up rules and then claim that disobeying them is a sin. What a corruption of the teachings of Jesus. What a hideous control tactic. Claim authority from God to determine who they let into heaven and who stays out and then threatening anyone who doesn't comply with eternal damnation of their soul.
The biggest irony is that Catholics are the first ones to shriek about how Protestants can't say who is saved and who is not and not only do Catholics do the self-same thing, but actually go on to tell others that they not only have the power to say who is saved and who isn't, but they even claim the POWER to determine who gets in and who doesn't.
They put themselves in the position of telling God what to do.
What an abomination before God.
Power corrupts and absolute power corrupts absolutely. That probably explains most of Catholic church history.
"..."This is my body" (Luke 22), you don't believe that."
And you also take this literally? JOHN 10:7 So Jesus again said to them, "Truly, truly, I say to you, I am the door of the sheep.
Truly, truly, Jesus is a door.