Why are Dems attacking Amy Barrett about being a Catholic yet Biden is Catholic like wtf?

I would guess because Barrett has a reputation for using her religion to decide cases instead of the law and constitution.

https://www.foxnews.com/politics/pelosi-says-quest...

Speaking out against her radical views is hardly attacking. She has no right to control the mass of Americans in the palm of her hands as though she be God. We need moderates on the courts. 

Biden understands that his religious beliefs should not dictate how the government runs. Barret wants to make rulings based on her religious belief, not the Constitution like she is supposed to. That is the difference.