First, I'd like to say that I praise God for your site. I use it for my Bible study and it has really helped me a lot.
Now the question I have is whether or not it is sinful for me to be in a church that is teaching we should only use "one cup" during the Lord's Supper. I have been taught that congregations that use multiple cups are lost and it's a salvation issue because we are not doing what the Lord instructed. I have read your Q & A on The Lord's Supper and agree.
Can you please elaborate on the supper as far as which part was done under the Law of Moses and exactly when the New Testament came in affect.
The Lord's Supper was instituted with multiple cups. "Then He took the cup, and gave thanks, and said, "Take this and divide it among yourselves; for I say to you, I will not drink of the fruit of the vine until the kingdom of God comes." And He took bread, gave thanks and broke it, and gave it to them, saying, "This is My body which is given for you; do this in remembrance of Me." Likewise He also took the cup after supper, saying, "This cup is the new covenant in My blood, which is shed for you"" (Luke 22:17-20). What confuses some is that Jesus used a common figure of speech, talking about "the cup" while each a portion of the drink in front of each of them.
If a church only uses one cup, while not required, it does not cause any harm. The Lord's Supper still can be observed. If the only thing being done differently is this one item, there is no harm staying.
The Passover was done under the Law of Moses. But the Lord's Supper was not a part of the Passover. There is nothing mentioned of it in the Old Testament. Paul mentions in I Corinthians 14:25 that the Lord instituted it after the Passover was completed.
The New Testament came into effect when Jesus died on the cross. "For where a covenant is, there must of necessity be the death of the one who made it. For a covenant is valid only when men are dead, for it is never in force while the one who made it lives" (Hebrews 9:16-17). This is also the same time that the Old Testament ended. "When you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, having canceled out the certificate of debt consisting of decrees against us, which was hostile to us; and He has taken it out of the way, having nailed it to the cross" (Colossians 2:13-14). It was necessary to end the Old Testament so that the Jews under that law could be joined to the law of Christ. "Therefore, my brethren, you also were made to die to the Law through the body of Christ, so that you might be joined to another, to Him who was raised from the dead, in order that we might bear fruit for God" (Romans 7:4).