Its sinful because of the difference of what the Catholic Church teaches and what the Reformation Church’s teaching, to Protestants its bread and grape juice, its just a symbol , and as Catholics we don’t receive a “Symbol” the Catholic Church teaches that when a priest celebrates Mass he has the power to turn the bread and wine (not bread and grape juice) into the Body and Blood of Christ. John 6: 53 -58
Line 53 starts off by saying. Jesus said to them, Amen, Amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you,
Summary.
As Catholics we can only receive the Body and Blood of Christ as commandeth to do so by Our Lord, which is called Transubstantiation , to receive anything else is incorrect, and why would you not want to eat the real thing , it would be like asking someone do you want to eat meat, or a plastic version of meat.
Now some of his Disciples found this teaching to hard and walked away, and Our Lord said to his twelve are you going to walk away as well, to which they replied "Master to whom would we go to , you have the words of Eternal Life.
Please read the whole of John Chapter 6 : 51-58.
An interesting book to invest in would be a book called "Catholicism for Dummies- it gives all the Catholic teaching in easy terms good for Converts and Catholics who are weak on there faith.
Hope the above helps.