From the pen of Jerry Bridges:
In our Christian circles, the word fellowship has come to mean little more than Christian social activity. It may mean the exchange of pleasantries over coffee and cookies at church, or the social functions of our high school or campus ministry groups. This is not the meaning of fellowship in the New Testament. The first occurrence of the word fellowship in the New Testament occurs in Luke’s account of the beginning of the New Testament church on the Day of Pentecost. As a result of Peter’s sermon, about three thousand people believed in Christ. Luke says of them that “they devoted themselves to the apostles’ teaching and to the fellowship, to the breaking of bread and to prayer” (Acts 2: 42). We’re not surprised that these new believers devoted themselves to the apostles’ teaching and to prayer. But to fellowship? It would seem strange to include fellowship along with teaching and prayer if fellowship meant no more than Christian social activity. Or consider the words of the apostle John in 1 John 1: 3: “What we have seen and heard we proclaim to you also, so that you too may have fellowship with us; and indeed our fellowship is with the Father, and with His Son Jesus Christ” (NASB). In both Acts 2: 42 and 1 John 1: 3, the New English Bible translates koinnia as “sharing a common life.” This is the most basic meaning of koinonia, or fellowship. It is sharing a common life with other believers — a life that, as John says, we share with God the Father and God the Son. It is a relationship, not an activity.
Those first Christians of Acts 2 were not devoting themselves to social activities but to a relationship — a relationship that consisted of sharing together the very life of God through the indwelling of the Holy Spirit. They understood that they had entered this relationship by faith in Jesus Christ, not by joining an organization. And they realized that their fellowship with God logically brought them into fellowship with one another. Through their union with Christ, they were formed into a spiritually organic community. They were living stones being built into a spiritual house (see 1 Peter 2: 5), fellow members of the body of Christ. As William Hendriksen said, “Koinonia, then, is basically a community-relationship.” 1 It is not primarily an activity; it is a relationship. It is this spiritually organic relationship that forms the basis of true Christian community. It is not the fact that we are united in common goals or purposes that makes us a community. Rather, it is the fact that we share a common life in Christ. There are many organizations, both secular and Christian, whose members work together to pursue common goals. Some of these groups may call themselves communities. But biblical community goes much deeper than sharing common goals, though it ultimately involves that. Biblical community is first of all the sharing of a common life in Christ. It is when we grasp this truth that we are in a position to begin to understand true community.*
*Bridges, Jerry (2012-09-14). True Community: The Biblical Practice of Koinonia (Kindle Locations 89-113). Navpress. Kindle Edition.