Brainwashed is a very, very good description of many Fundamentalists.
Another good description of them is insecure, wounded people who have not fully healed and who lash out in fear and prejudice.
Others are wonderful, open, deeply Christian, loving people who genuinely love the Lord and are our (doctrinally separated) brothers and sisters in Christ.
The ones who are not like this will actually listen when you explain your faith to them, and will see that while we may disagree, both Protestants and Catholics are truly Christians who are saved by grace.
There is a Fundamentalist on this board who actually went out of her way to send me an email explaining to me that “she was born again” and implied that I must not understand that because I am Catholic.
I was happy to correct her mis-understanding of my faith and my relationship to Jesus and explained to her that I understood what she was talking about, and I am, but am also Catholic.
She then began telling me that Catholicism is wrong because of infant baptism, the real presence in the Eucharist, and virulently argued that we WORSHIP STATUES

(This is in a thread here.)
She refused to listen to a word that I said when I explained her misunderstanding, and insisted that she knew what was in my heart better than me.
This is sad, ignorant, brain-washed arrogance that is at its roots full of fear and prejudice. She WON’T actually debate any of this because brain-washed people aren’t open to having their beliefs challenged.
What is so sad is that they are Christians… they do wish to follow the Lord. It is hard for me to not be very angry and disgusted at people like this. This is an attitude I must keep in prayer before the Lord to be transformed into a good witness for Him.