The Catechism in Questions & Answers

621. What does “communion” signify in the divine service?

“Communion” in divine service is understood as the fulfilment of the words of Jesus Christ: “For where two or three are gathered together in My name, I am there in the midst of them” (Matthew 18: 20).
This “fellowship” in the divine service also refers to the fact that the believers worship, praise, and give thanks to God together. Thus they also have fellowship with one another.