Key NT terms for fellowship are koinonia and echo. In the NT, echo is used of the intimate fellowship of sexual intercourse (Mt 14:4). It also refers to ‘having’ or being possessed by an evil spirit (Mt 11:18; Lk 5:33) and for having true fellowship with God (2 Jn 9). Remember Rom 8:9, 14 where ‘having’ the Holy Spirit means to be led or possessed by Him (Brown, NIDNTT, v 1, p 638). Our fellowship with God is the basis for our having fellowship with other Christians.
The NT commands us to regularly ‘associate,’ ‘communicate,’ ‘fellowship,’ and ‘participate,’ with our ‘partners’ and ‘companions’ in the faith as a way of persevering in our faith instead of falling away from Christ (Heb 10:23-26).
Christian fellowship is a key way God gives us grace. For instance, 1 Cor 16:18 (Phillips-1st ed) says that Paul’s fellowship with certain mature believers was like a tonic for him which was morally invigorating and restorative. Their fellowship provided him with mental relief, comfort, rest, relief from anxiety, encouragement, peace, good cheer, happiness, and helped to mend his relationship with the Corinthians. Being in fellowship reminds us that we are fellow citizens of heaven, called to live as citizens of heaven, and that the Lord takes careful and favorable note of those who often talk with other believers about Him. (Eph 2:19 with Mal 3:16, AB).
Let us seek out good Christian fellowship where we can know people well enough to show love to them and receive love from them.
Instead of being timid, learn to be appropriately bold.