1 John begins with the confirmation (nothing new here) that Man is supposed to have communion (fellowship) with God and like minded believers (v.1-4).
John then goes on to explain v.5-6 how God is light (Holy and unblemished) and we are to walk in that light. In fact verse 6 makes it clear we MUST walk in the light to have fellowship with God.
But if we walk in the light, – Note the “if” not all “christians” walk in the light. They are believers in name only.
as he is in the light, – If the light of Christ that is the Fruit of the Spirit is not shining through you are not walking in the light.
we have fellowship one with another, – The only way to have fellowship with one another is to be in the light.
and the blood of Jesus Christ his Son cleanseth us from all sin. The only way to be in the light is to be truly and FULLY redeemed by the Blood of Christ.
Listen, you cannot walk with God or fellowship with like minded believers if you do not know God. Think about it a minute; we are to be intimate with God, telling Him our most guarded secrets and our deepest desires. Would you do that to another human whom you knew nothing about, a stranger?
If we call ourselves born again, believer, Christian, child of God whatever label you prefer you must get to know your Father in heaven to Walk in His Light.
I offer the following link for your edification:
Take time today to get to intimately know God and enjoy true fellowship and joy with the Triune God and fellow believers.