1 Corinthians 10:21

Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils (KJV)

Then: Paul brings the Corinthians minds back to the Lord's supper, but this time to the wine rather than the bread. Still it's the same as verse 20 --a flat out statement. Believers cannot please a jealous God, the one true Lord and worship anywhere else. The commentators find no significance in the order of Paul's points concerning the bread or the wine. Both Hayes and Baker agree that Paul just has more to say about the bread as the body than than he has to say about the cup (Baker, p. 149; Hayes, p. 167).

Now: Paul's principle and directive is as clear today as in the first century. What he says matches Jesus teaching that no one can serve two masters. Today's Christians may need to work harder to discern the evidence of idolatry in their lives, but it must be done.

Extra source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.
First Corinthians by Richard B. Hayes (John Knox Press, 1997) on line at http://books.google.com/books?id=M_PC0PAs3VYC&dq=1+Corinthians+and+Richard+B.+Hays&printsec=frontcover&source=in&hl=en&ei=2oPcStDCIIvf8Aaq0Ki3BQ&sa=X&oi=book_result&ct=result&resnum=11&ved=0CCMQ6AEwCg#v=onepage&q=1%20Corinthians%20and%20Richard%20B.%20Hays&f=false