An important factor underlying this misperception of Berkeley as being a "subjective idealist" has been overlooking the role of the mind of God in Berkeley's metaphysics. This error, too, can be understood to arise from a failure to place Berkeley in his proper historical context: as contemporary philosophers aren't likely to give God a very prominent place in their metaphysics, it is hard for them to recognize the place that He had in Berkeley's system. Even Boucher and Vincent, in their otherwise excellent introduction to British Idealism, mention in passing "Berkeley"s radically subjective view of experience" (2012: 63). But for Berkeley, human experience is certainly not "radically subjective": humans find themselves confronted inexorably with an external reality not of their own choosing.
Here I wish to offer a metaphor intended to clarify the objective nature of reality as Berkeley sees it: Berkeley's God is like the creator of the video game, Mind of God: from Genesis to RevelationTM, and all other conscious beings are akin to players in that game. (Note that this is only a metaphor, and as with all metaphors, if pushed too far it will yield ridiculous results. For instance, if you find yourself asking "What program debugger did God use?" or "How many lines of code did he write?" you have taken the metaphor too literally.)
For Berkeley, the ideas God had in programming Mind of God create the sights, the sounds, and the rules of the game. For the players in the game, these ideas are objectively real. If they travel down a road that comes to a fork, every one of them who is not hallucinatory will see the fork and have to choose the left or right path. And for someone who is hallucinatory, their attempts to neglect the fork will be thwarted. If a player tries to ignore the fact that in Mind of God there is a drop off of a tall cliff down to some jagged rocks immediately ahead of him, he will find his "game player" body smashed to bits on the rocks. If she attacks a monster too many levels above her rank, she will lose. If a player pretends he doesn't need to eat, he will see his life force points draining away. If a player tries to move something God has deemed immovable, she will fail.
Within the parameters set by God, the players are free: they may choose to fight a monster or not, to take a road heading east or one heading west, to unite with other players or to go it alone. They will also have their own judgments about the game: they may think a particular forest looks frightening, or be soothed by a seaside vista. But in so far as those judgments concern the actual "coding" of the game, they are susceptible to being tested against that reality, and being proven objectively true or objectively false. If the player believes that the frightening forest is filled with goblins, when he heads into it he will discover either that it actually is and his judgment was correct, or it actually isn't, and he was wrong.
Kenneth L. Pearce's remarks on the role of language in Berkeley's world are apropos here:
According to Berkeley, the perceived world is itself a language -- or, rather, a discourse in a language. Berkley intends this claim quite literally. It is the linguistic structure of the perceived world that our thought and speech about co-instantiation, physical causation, and other structural concepts aims to capture. In this way, I argue, Berkeley succeeds in preserving the common sense and scientific structure of the perceived world... Bodies can be regarded as a joint product of God's activity as speaker and our activities as interpreters and grammarians of nature. (2017, pp. 2-3)
My metaphor is just a more colorful way of making this same point, where the logical structure of the code for the video game takes the place of linguistic structure in Pearce's argument. And if Berkeley himself had ever played a video game, he too might have reached for the idea of a programming language, rather than a natural language, as his vehicle for expressing God's creative power. (There is the difficulty that Pearce insists that language is not a metaphor here for Berkeley, but that he means this literally: the world really is "God's language." That point will be explored further in the full paper.)