What is communion? Scripture explains that the Passover is the communion of the blood of Christ (1 Corinthians 10:16), that light has no communion with darkness (2 Corinthians 6:14) and that the saints have communion with God and Christ through the Holy Spirit (2 Corinthians 13:14). Some use the term to describe a relationship to God that Christians generally enjoy because of the grace of Christ. Others use the term when describing organizational Christian fellowship within the body of a unified Church.

Full text at - http://www.tomorrowsworld.org/commentary/communion-by-grace-alone