It is the doctrine I believed to be right .
It is the doctrine I believed was right.
It is the doctrine I believe is right.
All three of these sentences are grammatical, and they all mean the same thing.
Further, all three lack the same thing -- a relative pronoun marking a relative clause:
It is the doctrine that/which I believed to be right.
It is the doctrine that/which I believed was right.
It is the doctrine that/which I believe is right.
All three of these sentences are grammatical and synonymous as well.
Notice also that in all three relative clauses, the relative pronoun, whichever it is,
is extracted from a clause below the believe clause; so let's reconstruct that originals:
- I believed the doctrine to be right
- I believed (that) the doctrine was right
- I believed (that) the doctrine is right
Once again, all grammatical and all synonymous. The relative clause formation rule extracts the doctrine and then deletes the relative pronoun because it's not the subject of its relative clause
(I is already the subject).
This leaves a gap between I believed and its object clause, which is reduced to a verb phrase only.
So why are there three variants? Because believe can take an infinitive complement
- Max believed Freddie to be guilty of the murder.
I believed the doctrine to be right.
as well as a tensed that-clause, of either tense:
- Max believed (that) Freddie was guilty of the murder.
I believed (that) the doctrine was right
- Max believed (that) Freddie is guilty of the murder.
I believed (that) the doctrine is right
and here all the clause-marking that's have been deleted.
They can't occur with an infinitive, only with a tensed clause.