John in this Epistle (The word "epistle" means a "letter") is revealing the criteria one must have in order to be in fellowship which God.
Only a saved, born-again Christian can be in fellowship with the Lord.
This does not mean that the Christian has lost his salvation, but rather has lost his personal communion with God and the blessings of that relationship . The relationship with God begins when one believes and receives Jesus Christ as his Savior and is an eternal relationship. Once one becomes a child of God the relationship with God is established. A believer cannot lose his relationship with God as a child of God but he can get our of fellowship with the Lord.
If a person professes to know God, and in truly is saved there will be evidences of his relationship with God that can be seen in his life.
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