"Communion" is a good word. What do you think when you hear it? Maybe an ordinance of the church? Perhaps an archaic way saying relationship? Or even some mystical ambiguity connected to transcendence?
Communion is one of the few words in the English language that has a general meaning but maintains a sanctified use. "To speak a little of it in general," John Owen writes, "Communion relates to things and persons. A joint participation in any thing whatever, good or evil, duty or enjoyment, nature or actions. . . . (Works.II.7). In other words, communion most bascially is what's happening when we cheer on our favorite team with a group of friends.
But that's not the way we really use the word. We call those parties. And notwithstanding the joy aspect of parties, communion is about God — the one, true, personal God in three persons, Father, Son, and Spirit.
Communion is God's communication to us coupled with our response to him — all in such a way that he's glorified and we're glad. Communion hopefully does happen when we do the Lord's Supper, but it's not limited to that event.
In his essay from Crossway's new Understanding Scripture, John Piper explains,
Communion refers to God's communication and presentation of himself to us, together with our proper response to him with joy. We say "with joy" because it would not be communion if God revealed himself in total wrath and we were simply terrified. That would be true revelation and a proper response, but it would not be communion.
Communion assumes that God comes to us in love and that we respond joyfully to the beauty of his perfections and the offer of his fellowship. He may sometimes come with a rod of discipline. But even in our tears, we can rejoice in the Father's loving discipline (Hebrews 12:6–11). Communion with God may lay us in ashes or make us leap. But it never destroys our joy. It is our joy (Psalm 43:3). (46)
So if this is what communion is, what about why it is? What's the purpose behind it? Pastor John writes,
Communion with God is the end for which we were created. The Bible says that we were created for the glory of God (Isaiah 43:7). Yet glorifying God is not something we do after communing with him, but by communing with him. Many human deeds magnify the glory of God's goodness, but only if they flow from our contentment in communion with him. (46)
But how? Communion is this glorious fellowship with God that fulfills the purpose of our existence. Yet how is it possible? Owen once more, "By nature, since the entrance of sin, no man hath any communion with God. He is light, we are darkness; and what communion hath light with darkness?" (6). The disparity here is incalculable. Consider us: sinners; and God: infinitely holy. How in the world might we commune with him?
The answer of the Bible is that God himself took the initiative to be reconciled to his enemies. He sent his Son, Jesus Christ, to die in our place and bear the curse that we deserved from God. "Christ redeemed us from the curse of the law by becoming a curse for us" (Galatians 3:13). So the wrath of God that we deserved fell on Christ (Isaiah 53:4–6, 10). Because God gave Christ as our substitute, we can be reconciled to God and enjoy peaceful communion with him. (47)