1 Corinthians 10:16-17 NKJV 

The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? For we, though many, are one bread and one body; for we all partake of that one bread.

The Three Unions in Holy Communion

I. Bread and wine with body and blood.

II. Sinners and God.

III. Believers with one another.