The Test of Fellowship (Part 1)
1 John 1:5-6
God is free from darkness (5b)
“…and in Him is no darkness at all.”
This added assertion that there is no darkness in God, not even the tiniest or minutest speck of darkness stresses the absoluteness of His nature as pure and holy light.
In God’s being there is not a single trace of darkness.
Keep this in mind, that John never implies that the darkness is simply the absence of light. John believes that darkness has a moral quality. This moral quality that defines darkness is in direct opposition to the moral character and quality of God.
This might not mean much to you but to the pagans and especially the Gnostics, this was a startling statement.
George G. Findlay wrote:
“They [he means the Romans and Greeks] had gods that could cheat and lie, gods licentious and unchaste, gods spiteful and malignant towards men, quarrelsome and abusive toward each other. They had been accustomed to think of the Godhead as a mixed nature like their own, only on a larger scale-good and evil and cruel, pure and wanton, made of darkness and light.” 
When men create their own god they create them in their own image. So the gods of men condone evil or allow them to live as they please and might eve participate in sin also.
But the truth of the matter is God cannot have fellowship with anything or anyone that does not share a moral likeness to Himself. God cannot condone or have fellowship with anything that is contrary to His nature.
So, John makes it very clear from the beginning of His letter that there exists two, completely separate spheres or realms that cannot be mixed in any way shape or form. Light and darkness are two distinctly separate spheres that cannot be mixed. They do not overlap.
The Christian life is viewed as fellowship.
This claim to fellowship is tested on practical grounds.
The first test by practical grounds is that of the necessity of moral likeness.
The first truth that we learn about this moral likeness is that:
God is the standard.