When you canonize a person, you put him on a pedestal — in other words, you think he's so wonderful that he can do no wrong.
You might canonize your grandmother after she dies, choosing to remember her as a saint who never raised her voice and was kind to everyone. When the Roman Catholic Church canonizes someone, it has a slightly different meaning. The Church canonizes people who have performed miracles and are declared — literally — to be saints. The literal meaning is "place in the canon of saints," and it comes from the Latin canon, "church rule."