Goldstein explains all the slogans of the Party in such a way that it seems clear, that he wrote them, that he was the founding father.
He is not Anti-Party. He is not a traitor. He gives its basis, the reasons for the various actions. It is inescapable that everything that exists in the world inhabited by Winston Smith was formulated by him.
Yet, Goldstein is considered the enemy of the state, and the population is impelled to hate him.
To support the plans and protocols created by Goldstein, to live in the world he created, yet to have him as an enemy and traitor is probably the clearest evidence of doublethink.