To appreciate the full meaning of the word-group in the New Testament that conveys the nature and reality of Christian fellowship (i.e., the noun koinonia [koinwniva], the verb, koinonein [koinwnevw], and the noun koinonos [koinwnov"]) as used in the New Testament, it is necessary to be aware of two fundamental points.
First, the fact and experience of Christian fellowship only exists because God the Father through Jesus Christ, the Son, and by/in the Spirit has established in grace a relation (a "new covenant") with humankind. Those who believe the gospel of the resurrection are united in the Spirit through the Son to the Father. The relation leads to the reality of relatedness and thus to an experienced relationship (a "communion") between man and God. And those who are thus "in Christ" (as the apostle Paul often states) are in communion not only with Jesus Christ (and the Father) in the Spirit but also with one another. This relatedness, relationship, and communion is fellowship.
By his sacrificial death and glorious resurrection/exaltation, Jesus Christ brought into being a new creation, a new order, and a new epoch. Though this new situation will only be present in fullness at the end of this evil age, it is a reality now on this earth. Christ exercises his relation in this new creation in and through the controlling and liberating Holy Spirit, whom the Father sends in the name of Christ. Thus to be "in the Spirit" is also to be "in Christ." And this is another way of saying that Christians who are baptized into Christ and given the gift of the Spirit are dynamically related to the Father through the incarnate Son in and by the Spirit of the Father and the Son. On the basis of this relation there is fellowship for Christians both with God and with each other.
In the second place, it is probably best not to use the word "community" as a synonym for "fellowship." The reason for this is that in modern English "community" presupposes "individualism" and thus carries a meaning that is necessarily foreign to biblical presuppositions since individualism (i.e., the thinking of a human being as an "individual" and as the basic unity of society) is, technically speaking, a modern phenomenon. So "community" seemingly inevitably today usually refers to a group, body, or society that is formed by the coming together of "individuals" in a contractual way. The emphasis is on the initiative of the "individuals" and on the voluntary nature of the group thus formed. In contrast, koinonia [koinwniva] has its origin in a movement out of the internal, eternal relation, relatedness, and communion of the Godhead of the Father and the Son and the Holy Spirit. Koinonia [koinwniva] for baptized believers is thus a participation within human experience of the communion of the living God himself.
General Background. In the colloquial Greek of the New Testament period, koinonia [koinwniva] was used in several ways. It was used of a business partnership, where two or more persons share the same business and are thus closely connected in work. Also it was used of marriage, of the shared life of two persons, a man and a woman, together. Further, it was sometimes used of a perceived relatedness to a god, such as Zeus. Finally, it was used to refer to the spirit of generous sharing in contrast to the spirit of selfish acquiring.
Much of the use of the word group koinonia [koinwniva], koinonein [koinwnevw], and koinonos [koinwnov"] in the New Testament corresponds to general Greek usage. Thus the fellowship and sharing are religious or specifically Christian only if the context requires this meaning. For example, in ac 2:42 we encounter the word Koinonia [koinwniva] and read that the new converts continued in "the apostles' teaching and to the fellowship." Here it is a normal meaning adapted to Christian usage. Then the verb, koinonein [koinwnevw], is found in Hebrews 2:14 with an ordinary, general meaning: "children share flesh and blood." Likewise, koinonos [koinwnov"] occurs with the meaning of "partner" in Luke 5:10" [James and John] Simon's partners. "