A judge's appointment letter alone makes them a judge who can start deciding cases immediately.

The person cannot enter upon office or exercise judicial powers until they have taken and signed the oath as described in Article 219.

Any officer can administer this oath.

It must be taken before the Governor of the State, or a person specifically appointed by the Governor for that purpose.

The Constitution of India

Article 219

Oath or affirmation by Judges of High Courts

Every person appointed to be a Judge of a High Court shall, before he enters upon his office, make and subscribe before the Governor of the State, or some person appointed in that behalf by him, an oath or affirmation according to the form set out for the purpose in the Third Schedule.§

Why this exists

The framers wanted every judge to publicly commit—before taking office—to uphold the Constitution and perform duties without fear or favour. Requiring this before the Governor (the constitutional head of the State) gives the act formal, ceremonial weight and marks the legal start of the judge's authority, mirroring similar oath requirements for the President, Governors, and Supreme Court judges.

Common misconceptions

Myth: A judge's appointment letter alone makes them a judge who can start deciding cases immediately.
Fact: The person cannot enter upon office or exercise judicial powers until they have taken and signed the oath as described in Article 219.
Myth: Any officer can administer this oath.
Fact: It must be taken before the Governor of the State, or a person specifically appointed by the Governor for that purpose.