The word "fellowship" is used in 1 John a total of four times, all in the first chapter.

What we have seen and heard we announce to you too, so that you may have fellowship with us (and indeed our fellowship is with the Father and with his Son Jesus Christ).1 John 1:3

If we say we have fellowship with him and yet keep on walking in the darkness, we are lying and not practicing the truth.1 John 1:6

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

There are two general views of what this fellowship is referring to (labels my own):

1. Right living, Good Christian View - This view states that, by "fellowship", John is referring to living in obedience, without sin. Some in this camp believe this "fellowship" is a state of sinless perfection, while others believe it is period of time in someone's life defined by maturity, growth, and obedience. This view states that the "fellowship" John speaks of is being a "good Christian", "walking closely" with God. The opposite of a Christian "in fellowship" would be a "carnal Christian", or someone who, though saved, is living according to fleshly indulgences.
2. Salvation View - This view states that the "fellowship" John is referring to is any real association or relationship with God. That is, John is talking about Salvation. Whereas the right living view says that John is speaking of walking "closely" with God, the salvation view says that John is speaking of whether a person has any walk with God whatsoever.

Scripture indicates that the salvation view is correct. John is speaking about whether or not the one professing Christ is actually saved. John Stott summarizes, "'Fellowship' is a specifically Christian word and denotes that common participation in the grace of God, the salvation of Christ and the indwelling Spirit which is the spiritual birthright of all believers."¹.

The Greek

The Greek word translated "fellowship" in these verses is koinonia, which occurs 19 times in the New Testament. A translation note in the NET Bible reads:

Or "communion"; or "association" (a reality shared in common, so in this case, "genuine association").

Strong's (Greek word 3126) describes koinonia as "fellowship, the close association between persons, emphasizing what is common between them; by extension: participation, sharing, contribution, gift, the outcome of such close relationships".

Many would agree with me that the fellowship John speaks of is not the kind of "fellowship" often times indicated by use of the same English word today. We might use the word to describe enjoying someone's company, or having a feeling of companionship with someone. But this is not the meaning or use of the Greek word in 1 John. According to Friedrich Hauck, in Theological Dictionary of the New Testament, koinonia represents a partnership, such as a business relationship, a marriage, or even sex. While the word does seem to have multiple applications, general usage and specific context do away with the ambiguity.

Elsewhere Usage

because of your participation in the gospel from the first day until now.Philippians 1:5

God is faithful, by whom you were called into fellowship with his son, Jesus Christ our Lord. 1 Corinthians 1:9

Is not the cup of blessing that we bless a sharing in the blood of Christ? Is not the bread that we break a sharing in the body of Christ?1 Corinthians 10:16

and when James, Cephas, and John, who had a reputation as pillars, recognized the grace that had been given to me, they gave to Barnabas and me the right hand of fellowship, agreeing that we would go to the Gentiles and they to the circumcised.Galatians 2:9

The verb form (koinoneo) is used by John here:

because the person who gives him a greeting shares in his evil deeds.2 John 1:11

There are many other appearances of the word in its many forms, some with different meanings. In some instances, the word is accompanied by two parties and an object. That is, Party 1 shares Object with Party 2. For example, Paul says that the Macedonian believers wanted to share with the Corinthian believers in helping the saints (2 Corinthians 8:4). In many other cases, as in 1 John, the word appears without this context, and so the meaning is more ambiguous.

Context of 1 John

Looking at the immediate context, we see the repeated mention and emphasis of eternal life in 1 John 1:1-3. John's closing statements in chapter 5 put the whole book into the context of discussing salvation. He mentions the identification of those who believe that Jesus is the Christ (v1), carries this identification further (v2-3), and identifying specifically "everyone who has been fathered by God" (v4). These faithful conquerors are mentioned again (v5), John picks up speaking about those who believe a few verses later and contrasts them with those who don't (v10), and John specifically mentions eternal life multiple times in verses 11 and 12. Obviously, John is concerned with eternal life and true believers. He comments in verse 13:

I have written these things to you who believe in the name of the Son of God so that you may know that you have eternal life.1 John 5:13

John here not only continues the discussion of eternal life, but specifically tells us that it is his purpose in writing. Throughout the entire book of 1 John, we read about eternal life. Back in chapter 1, which includes our word koinonia, John mentions "the word of life" (v1), "the life" and "the eternal life" (v2), and then says that it is this same thing that was seen and proclaimed (v3, cf. v1) that gives John reason to write to his audience so that they may have fellowship with him and those with him.

The "fellowship" John is writing about is also given meaning by his qualification in verse 3. John speaks about having fellowship with his audience, and then says that his fellowship is with the Father and the Son. These two relationships seem to be the same thing. In fact, John seems to be explaining the significance of his fellowship with his audience by explaining his fellowship with God. The question is, "Why does having fellowship with John matter?" The answer is, "Having fellowship with John is having fellowship with the Father and the Son." The certainly seems to refer to the brotherhood that all believers share with each other in Christ. John's focus on "life" in the first few verses may support this understanding, as John (Party 1) is speaking specifically about sharing with his audience (Party 2) in the eternal life that is in Jesus (Object).

Self-Stated Purpose

It is important to note that John himself tells us why he is writing; "so that you may know that you have eternal life" (1 John 5:13). John gives a similar statement of purpose at the end of his gospel:

But these are recorded so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.John 20:31

Both statements talk about eternal life (as does the entire book of 1 John). The gospel of John is written so that the audience might believe and thus have eternal life, the epistle is written so that the audience might know that they have eternal life. The first is about obtaining salvation, the second is about assurance of salvation.

All through 1 John, John gives us tests to see whether we have eternal life. The tests involve practicing righteousness, loving fellow believers, and believing in Jesus. If the rest of 1 John is about salvation, why would the first chapter be any different? In fact, the first chapter tells us that one sign of having eternal life is a right attitude towards sin. Again, it's about salvation, not about how rightly we walk.

Lastly, the language of 1 John leaves no room for variance within realms of fellowship. If the right living view were correct, we would expect varying degrees of fellowship. A godly man today may be in good fellowship with God, but perhaps his fellowship with God is not as great as that of Paul or John. But John's language doesn't leave any room for degrees of fellowship: your'e either in or you're out. Thus, to adhere to the right living view of fellowship in 1 John, one would almost necessarily have to agree with Wesley that the fellowship John is writing about is a state of sinless perfection. While it is true that "you're either in fellowship or you're out of fellowship", John is not talking about whether your Christian walk is perfect or less than perfect, but whether you even have a Christian walk to begin with.

Notes

1 John R. W. Stott, The Letters of John, 2nd ed. (Grand Rapids: Eerdmans, 1988), pp. 68-69.