When Paul concludes his letters to the churches, it is easy to pass by the last words. For example, 2 Cor. 13:13 ends with these theologically dense words.
May the grace of the Lord Jesus Christ, and the love of God,
and the communion of the Holy Spirit be with you all.
The word 'communion' is the Greek word koinonia. It can be translated as 'communion', 'fellowship', 'participation', and 'intimacy' (and even 'intercourse'. See here.)
Communion connotes closeness, a sharing of what we have in common. Apply this to the Holy Spirit and we have something deep, and profoundly relevant to life.
Dallas Willard picks up on this as he writes:
"While communication with God would be a stretch for many, there is still more. In the progress of God’s redemptive work, communication advances into communion.
Communication often occurs over a certain distance, even amidst possible opposition. We can still communicate with those with whom we are at war. God communicates with us even while we are his enemies, dead in our sins. When communication between two people rises to the level of communion, there is a distinctness but also a profound sharing of the thoughts, feelings and objectives that make up our lives. Each recognizes the thought or feeling as his or hers, while knowing with joy that the other is feeling or thinking in the same way."
(Willard, Hearing God Through the Year: A 365-Day Devotional, p. 223)