1st John 1:5 (CSB) This is the message we have heard from him and declare to you: God is light, and there is absolutely no darkness in him.
Recall that John’s first epistle is all about “life”: “the life” (v.2), “the eternal life” (v.2), “the word of Life” (v.1).
Therefore it is fitting that the apostle’s first verse in which he begins to unfold the eternal word of life should begin with the Light of God:
John 1:4 (CSB) In him was life, and that life was the light of men.
God is the light of life from whom all living creatures draw their breath. This is why in the prologue (v. 1-4) of John’s first epistle, he uses focused repetition to emphasize life and then moves directly into our restored fellowship with the Father through Jesus Christ.
This is why Jesus says this:
John 17:3 (CSB) This is eternal life: that they may know you, the only true God, and the one you have sent—Jesus Christ.
Humanity was created to be children of the light, to flourish in God’s light of life. But we sinned in pursuit of something that did not belong to us (the knowledge of darkness) and our unimpeded fellowship with God was fractured because we took darkness into our hearts. The darkness of death has no place in God’s presence and we separated ourselves to pursue the things of death (sin). Though Satan transforms himself into an angel of light to deceive and draw us away from the light, Satan is the mediator of darkness.
But darkness cannot overcome the light. Therefore, God took on flesh and sent his unique Son to dispel the darkness and shed light into the hearts of those who believe:
John 1:1-5, 9-10, 14, 18 (CSB) In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was with God in the beginning. 3 All things were created through him, and apart from him not one thing was created that has been created. 4 In him was life, and that life was the light of men. 5 That light shines in the darkness, and yet the darkness did not overcome it… 9 The true light that gives light to everyone, was coming into the world. 10 He was in the world, and the world was created through him, and yet the world did not recognize him… 14 The Word became flesh and dwelt among us. We observed his glory, the glory as the one and only Son from the Father, full of grace and truth… 18 No one has ever seen God. The one and only Son, who is himself God and is at the Father’s side—he has revealed him.
In plain terms, the fellowship that Christ gives us with the Father is the glory of fellowship without any darkness of separation impeding us from our Father. It is the eternal light of life that Christ restores in the heart of the redeemed:
2nd Corinthians 4:3-6 (CSB) But if our gospel is veiled, it is veiled to those who are perishing. 4 In their case, the god of this age has blinded the minds of the unbelievers to keep them from seeing the light of the gospel of the glory of Christ, who is the image of God. 5 For we are not proclaiming ourselves but Jesus Christ as Lord, and ourselves as your servants for Jesus’ sake. 6 For God who said, “Let light shine out of darkness,” has shone in our hearts to give the light of the knowledge of God’s glory in the face of Jesus Christ.
God separated the light from the darkness by an eternal decree of his holiness; those who would seek the things of darkness cannot have any part in God. But those who seek the light of life in God through Christ will have no darkness of separation from him but overflow in the joy of eternal fellowship with their Creator!
In essence, both the “life” (v.1-2) and “fellowship” (v.3-4) that John introduces his first epistle with are wrapped up in the term “light” in such a way that communicates the exclusively holy nature of the life and fellowship of God. True life has no shadow of death, nor any degree of separation in fellowship from God.
Biblical Reasons 