Berkeley says that we never experience matter, we only experience mind. That is, when you see a chair, you are experiencing your mind, not the chair. We can not infer that the chair exists outside a mind, because we never experience the chair... We only experience the mind...
God, being the wonderful guy that he is, would not create such a confusing dualism for people to live in... In other words, god would not confuse us by creating a mental, detached from the physical. Therefore, god would only create the mental, and we would be directly aware of its existence. So, everything in the world is a representation of gods mind. And since we experience things as mind, we experience the world directly, because the world is mind.
Furthermore, he is an empiricist because he believes that all knowledge comes from experience.