In our Christian circles, the word fellowship [Koinonia] has come to mean little more than Christian social activity. However, the first occurrence of fellowship in the New Testament is at the day of Pentecost at the beginning of the church.
“they devoted themselves to the apostles’ teaching and to the fellowship, to the breaking of bread and to prayer” (Acts 2: 42).
It would seem strange to include fellowship along with teaching and prayer if fellowship meant no more than Christian social activity.
“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” (1 John 1:3)
Do you think John means he has Christian social activity with the Father and His Son? That would be ridiculous. Fellowship in the Biblical context is not an activity but a RELATIONSHIP!
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.
Bridges, Jerry (2012-09-14). True Community: The Biblical Practice of Koinonia