Idiom for a person who is above any question

Solution 1:

is considered beyond reproach

beyond reproach Blameless, faultless, as in Jean's conduct at school is beyond reproach. The phrase employs the verb to reproach in the sense of "censure or rebuke," a usage dating from the early 1500s.
— The American Heritage® Dictionary of Idioms by Christine Ammer

reproach — [...] reprochen "to rebuke, reproach," from Anglo-French repruchier, Old French reprochier "upbraid, blame, accuse, speak ill of," [...]
— etymonline.com

EDIT: Originally this answer was simply beyond reproach though, as commenters said, this implies subject is in fact perfect, hence added the prefix "is considered" as suggested by @jasper

Solution 2:

Above the law:

(idiomatic) Exempt from the laws that apply to everyone else.

[Wiktionary]

Your example:

The politician is above the law and people can't question his actions.

Solution 3:

There's an idiom that involves religion and could apply to your case:

Sacred cow

from Wiki and Collins

"someone or something that has been accepted or respected for a long time and that people are afraid or unwilling to criticize or question" (emphasis, mine)

With "Holy" there's Holy Cow, but that does not apply to your definition; it is just an exclamation to avoid blasphemy –from a Christian centric point of view– though it might be blasphemous for an Hindu.