What's a byterian?

When Weston was admitted to the NICU at Presbyterian Hospital (Dallas, TX) at birth, they gave the parents a backpack with a bunch of information in it.  Our friends' son had been in the same NICU almost exactly 2 years earlier.  I noticed later that he used that backpack at preschool at our church in Dallas.

I came across this backpack again after we moved to Austin and made it Weston's school backpack when he started All Saints last year.  It has some child's handprints and he seemed to like it well enough. (Not yet aware that he could request some other type of backpack.)

Anyway, now that he is getting into letters and learning that they spell things, he recently asked me what his backpack said.
"Presbyterian Hospital"
"Presbyterian Hospital" he repeated.
"Yes, that's where you were born."

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Several days pass and I hear him saying "Christbyterian Hospital"
"No, PRESbyterian"
"Christbyterian"
"No, PRESS. Like your seat belt buckle says PRESS."
"Presbyterian Hospital."

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Several more days pass and he says again "Christbyterian Hospital"
"No, PRESbyterian"
"Presbyterian. What's a byterian?"
"No, it's Presbyterian."... I can't manage to recreate the rest of the conversation, to summarize, I tried to explain that Presbyterian was like a kind of church and that the people from the church started the hospital and that's why it was called Presbyterian.  I do not think I was successful.

If only the hospital had changed their name to Texas Health Resources before 2008!!