This was a practice sentence on Languagezen.com. I thought the correct answer would be "Mi esposo piensa que es santo".
I've read the following advice about using articles with ser:
Don't use the article before an unmodified noun after a form of ser ("to be"), especially in reference to occupation, religion, affiliation or social status.
Normally, if the noun is modified, the article should be used.
Although saint is not a profession, nor a religious affiliation, it is a role that defines one's identity just as significantly as profession or religious affiliation, which I presumed to be the underlying grammatical principle here.
Also, the noun is not modified here (e.g. "un santo muy importante"), so that rule doesn't apply.
So is using "un santo" correct here, and if so, why?