I would expect the Catholic church, just like any religion or religious system to consider opposition to it a *grave sin*. It's a great tool for controlling the masses.
But heresy against CATHOLICISM is no sin at all. God doesn't mention it at all in Scripture.
And the Catholic church cannot just make sins up to slap on people and condemn them over.
Anyone who follows the word of mere men over the Word of God is in a bad way and will live to regret it.
Do you suppose they might regret it even more, to die by it?