A sentence: One may muse at length on what went on in Constantine's heart and mind with regard to the Christmas faith, but as an emperor he could not fail to appreciate the organizational and economic effectiveness of this particular church.
As an English major, I know when we refer to something impossible, COULD NOT DO for the present, COULD NOT HAVE DONE for the past, and I have read
http://oald8.oxfordlearnersdictionaries ... ionary/can
http://oald8.oxfordlearnersdictionaries ... nary/could
three times each, but the above sentence is unintelligible: COULD NOT FAIL should mean IT WAS NOT POSSIBLE FOR HIM TO FAIL, but definitely now it's not; could you tell me what this COULD means?