John's audience is in a dangerous situation.  They are being confronted with false teachers, "antichrists," who are trying to lead them astray.  John begins his letter by reassuring them that he and the other Apostles are trustworthy messengers.  They were with Christ, the Word of Life, from the beginning of His public ministry and have passed on in their writings what they witnessed.  Maintaining fellowship with the Apostles was a sure way for this troubled church to find fellowship with God.  Today, our churches do this by holding fast to their teaching, which is found in Holy Scripture.  Fellowship is not attained by our good works but when Christ condescends in the Apostolic word to bless His church.