The OT does not often mention fellowship. There exists a kind of covenant fellowship between God and Israel, with God as the senior partner. The closeness between them is expressed in Pss. 42 and 63. But the notion of fellowship (the Greek word is koinonia) comes into its own in the NT, especially in the epistles of Paul. Believers' fellowship is the unity in love effected first by baptism and then maintained by the community meetings for the Lord's Supper (1 Cor. 11: 17–34). Sacramental eating of the one bread is the appointed means for communicants to enjoy fellowship with the Risen Lord (1 Cor. 10: 16–17) and by its nature it excludes fellowship with demons (1 Cor. 10: 19–21).
The gospel of John has a principal theme of the love which believers must have for each other (John 13: 34) and which is illustrated by the allegory of the True Vine (John 15), of which disciples are the branches. Christian fellowship is rooted in the relationship of the Father and the Son (John 17: 20–23).