The “mystery of communion”, what does it mean?

The Church document uses a technical expression; “mystery of communion in Christ in the Spirit” provokes another question, what does “communion in Christ” mean? Why “in” and not “with” Christ?

The word “in” means, part of something internal. The word “with” is just an external accompaniment, an outsider in the construction of an English word. To understand it better, a story in the life of St. John Mary Vianney might help.

The famous French priest noticed with joy as a pastor that a village parishioner comes to the church at the so-called unholy hour, around one o’clock in the afternoon, to visit Jesus in the tabernacle of the parish church. After several days of curiosity, he approached the villager with a fatherly smile. He told his parishioner he was very happy and impressed to notice him every afternoon. But, “I didn’t see you move your lips in prayer.” The man humbly replied, “I do pray, Father, I just look at him.”

What does he mean? He looks at the tabernacle presence, not using, but seeing as Jesus sees, because he is in Jesus and Jesus is in him! He is almost intimately identified as Jesus! He is not completely identified as Jesus. But he cannot say like Jesus who said, “the Father and I are One!”.