In describing God as light, John is referring to His absolute moral purity and omniscience. In other words, there is no moral defect, nor is there a lack of knowledge in God. John’s point seems to be that the person and work of Christ, which is what is announced in the gospel message, is the light that brings us into fellowship with God, who is the light.

This is consistent with what Paul says in 2 Corinthians 4:3–6: “. . . For God, who said, ‘Let light shine out of darkness,’ has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ.” God’s Word in general is described as light (Ps. 119:105), but it is the gospel in particular that is the light that reveals God in fullness and brings us into fellowship with Him. John’s description of God as light in whom there is no darkness at all undergirds the fact of His utter otherness and therefore His inaccessibility to fallen creatures who exist in darkness.

The critical link for fellowship between creatures of darkness and a Creator who exists in absolute light is the incarnate Son of God. Without Christ, we remain in a state of darkness and alienation from the life of God. — Ken Jones