devil's advocate
Concise Oxford English Dictionary © 2008 Oxford University Press:
devil's advocate
▶noun
- 1 a person who expresses a contentious opinion in order to provoke debate or test the strength of the opposing arguments.
- 2 historical a person appointed by the Roman Catholic Church to challenge a proposed beatification or canonization, or the verification of a miracle.
'devil's advocate' also found in these Oxford entries:

