FELLOWSHIP

1 John 1:7. But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.

2 Corinthians 6:14. Be ye not unequally yoked together with unbelievers:  for what fellowship hath righteousness with unrighteousness?  And what communion hath light with darkness?

Amos 3:3 Can two walk together, except they be agreed?

I have heard fellowship described as, at least two fellows in the same ship going in the same direction.  Fellowship means to share things in common with another.  Only Christians share Christ in common and the life lived because of the Spirit living within us.  I think that is what is described in the above scriptures. 

A person who is a believer in Christ cannot have real fellowship with someone who is not a believer in Christ. A person living a righteous life cannot have real fellowship with someone who is living an unrighteous life.  As a believer you are part of the kingdom of light and as an unbeliever is part of the kingdom of darkness.  So the believer and unbeliever are really part of two very different kingdoms that are at war with each other.

When we walk with Jesus, who is the light, we can only have true fellowship with those who are also walking in that same light. This does not mean we cannot have friendships with or reach out to those not walking in light, otherwise we would not have any contact with unbelievers to share the light of the gospel with them so that they can be delivered from darkness. The light in us should overcome the darkness in them.