Who said Santa was not appropriate for Christians?

Answer: no one

Answer: Dr. Terry Watkins for one. See Link.

Answer: Dutch Protestants (in the US) did not like the celebration of Christmas linked with a Catholic Saint (Saint Nicholas); They Converted Saint Nicholas into a mythical Nordic Magician [Sint Klaes]; Saint -"Sint" = Santa + = Nicholas [clas] = Klaes = Clause