[aMorePerfectUnion]

“But communion with what? Not of “believers”

Yes. A fellowship together, remembering His sacrifice.

[Petrosius]

The cup of blessing that we bless, is it not a koinonia in the blood of Christ? The bread that we break, is it not a koinonia in the body of Christ? (John 10:16)

The Greek reads of the union with the body and blood of Christ, not of the fellowship of believers.