Mary offended the minister by playing spooky music that had no soul to it and hence was considered offensive and inappropriate to be played in church period.
I've been chasing grace/ But grace ain't easy to find

Because most organists wear shoes while playing. It makes it easier to find the correct notes.

Wanting to lick her toes in Church just wouldn't have been right.

The bare feet on the pedals were kinda disturbing.
Why.
++++++
Love means never having to say you're ugly. - The Abominable Dr. Phibes

The bare feet on the pedals were kinda disturbing.

Too much passion in her playing.

And lets face it, she LOOKED possessed while playing.

Well, it was very dark, "horror" sounding music. Most Christians I know believe horror to be evil and demonic by nature, so it's no surprise that music that sounds like it belongs in a haunted house wouldn't be welcome in a church. And plus, even if it wasn't scary sounding it is secular as well. Churches exist for the sole purpose of worshiping God so worldly music that has nothing to do with God really doesn't have a place in a church.

This scene is the key to the movie.
Once you understand this scene, you will understand everything else.

I think it was the combination of playing the eerie music and Mary being anti-social that made the minister fire her. She didn't quite fit in to the church's lifestyle.

The minister was just an old, stuffed shirt. I'll bet he ended up decrying rock music for being Satanic later in life.
Life is like a box of chocolates. A cheap, thoughtless, perfunctory gift that nobody ever asks for.

Actually, it's something in music called "dissonance" that irked the man of god. Quite frankly, Bach's Toccata and Fugue in D Minor sounds as if it were written by satan. And I noticed that he didn't want her to tickle the ivories for the church but she could hang around for spiritual guidance.

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This message has been deleted by the ghost from Carnival of Souls.

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He was mad because she was playing the theme music to Carnival of Souls.

She was playing ominous music, the music we hear right before he clamps down on her hands.
I thought the minister really overreacted to her playing the music, firing her!
"I'd say this cloud is Cumulo Nimbus."
"Didn't he discover America?"
"Penfold, shush."

I doubt anyone's checking these boars anymore but the devil's chord is my guess
http://www.forteantimes.com/features/articles/94/the_devils_chord.html
Cheers!
/)/)____./"""/')
))'_)

Lol..twelve bar blues.

The music was eerie and not acceptable to be played in his church. He commented on her not having a soul and more than likely he probably though the devil had put her in the trace she was in while she was playing. Honestly all organ music is eerie to me!

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I think it was more how badly she was playing that upset him.

Maybe he liked 12 bar blues?