In this particular part of A Treatise Concerning the Principles of Human Knowledge, Berkeley responds to the objections of those who argue that his subjective idealism—the notion that everything in the universe consists of ideas, not matter—is incompatible with a belief in God as traditionally conceived. As a bishop as well as a philosopher, Berkeley is understandably keen to refute such an objection, which he proceeds to do in paragraph 61 of part 1.
The argument that Berkeley seeks to dispose of is this: if the animal and vegetable life we see around us consists of nothing more than ideas, then why do they need to be so complex in their workings? Surely they could work just as easily without all their components. God could simply command them to work by a divine act of will.
With such a profusion of different bodies—both animal and vegetable—each with their own unique complex structures, how is it possible to discern any over-arching purpose, any final cause, to use the old Aristotelian terminology? If Berkeley is right, then it seems that there's no place for God in his philosophical system.
Berkeley responds in paragraph 61 by frankly admitting that his philosophy doesn't account for all the intricate workings of nature. But he quickly goes on to say that this is of no importance against that which can be proved a priori, that is to say on the basis of reasoning rather than through experience. And for him, the existence of God can be proved on such grounds.
He then turns the tables on those who maintain the existence of God's creation but not God himself. All of the characteristics of the many things that God creates, such as solidity, bulk, figure, and motion, ultimately have no activity in them; they cannot produce any effect in nature.
That being the case, those who suppose that created objects such as plants, animals, and human beings exist without being perceived, as the atheist does, are doing so to no purpose. The only use that is assigned to such objects is that they produce perceivable effects. And if we cannot perceive them, that must mean they can only be perceived by God.