God has spoken and He has communicated His Word to us. Firstly by revelation when God spoke to the writers. Secondly, through inspiration as the writers were divinely guided by God, in the process of writing His message. However, in order to understand God’s revealed and inspired message, illumination is required. This is where God causes the Bible to be understood by both the human heart and the mind.
Why do we need of illumination? Without illumination, the reader is blinded both by his nature, and by Satan. Just as a light bulb needs power in order to give light, so does the Bible need somebody to provide the power! The person, who does this illuminating, is God the Holy Spirit.