In my view, a teaching must satisfy the following conditions to be an essential, core Christian truth ...
These doctrines have ample evidence in ...
In my opinion, these are the doctrines which Christians must believe. If groups reject these, they are not properly called Christian.
The following doctrines are rejected by some. In my opinion, these ought to be essential. But for the sake of ecumenism we must allow non-Catholic Christians to reject these.
Quotes from the Bible with my comments ...
Is not the cup of thanksgiving for which we give thanks a participation in the blood of Christ? And is not the bread that we break a participation in the body of Christ? Because there is one loaf, we, who are many, are one body, for we all partake of the one loaf. Consider the people of Israel: Do not those who eat the sacrifices participate in the altar? . . . No, but the sacrifices of pagans are offered to demons, not to God, and I do not want you to be participants with demons. You cannot drink the cup of the Lord and the cup of demons too; you cannot have a part in both the Lord's table and the table of demons. (1 Corinthians 10:16-18, 20-21)
Those who partake of the Eucharist are united in the one body of Christ. Baptism also unites us in the body of Christ so the Eucharist is an extension of our baptism. By partaking of the Eucharist we are confessing our faith by our actions.
Paul is exhorting them to "flee from idolatry." (1 Corinthians 10:14) He is equating the Eucharist with food offered to demons in pagan sacrifices. Just as pagans are united with the demons when they eat the food offered to demons, just so, Christians are united with Christ when they partake of the Eucharist.
He also equated the Eucharist with the animal sacrifices of the Old Testament in which those who offer the sacrifice and then eat part of the offered animal participate in the sacrifice. Just so, those who partake of the Eucharist are partaking in the sacrifice of Christ. Therefore, the mass is a sacrifice in which Christ is offered (re-presented) as a sacrifice on our behalf.
For whenever you eat this bread and drink this cup, you proclaim the Lord's death until he comes. Therefore, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. A man ought to examine himself before he eats of the bread and drinks of the cup. For anyone who eats and drinks without recognizing the body of the Lord eats and drinks judgment on himself. That is why many among you are weak and sick, and a number of you have fallen asleep. . . . When we are judged by the Lord, we are being disciplined so that we will not be condemned with the world. So then, my brothers, when you come together to eat, wait for each other. If anyone is hungry, he should eat at home, so that when you meet together it may not result in judgment. (1 Corinthians 11:26-34)
In partaking of the Eucharist, we are proclaiming Christ's sacrificial death. Because of the significance of Christ's action, we must partake in a worthy manner or we will be insulting Christ and His sacrifice on our behalf. We should recognize Christ's body in partaking. People have gotten sick and died for not doing so as judgment from God. One of the things they were doing which was unworthy was being so hungry that they acted inappropriately during the Lord's supper.