In the eyes of the Catholic church it’s a very serious transgretion that leads to the denial of Holy Communion. In other eyes it’s a small white unleavened wafer about the size of a quarter. Why is it a big deal?

Because of the symbolism. The wafer in supposed to be the reincarnation of the body and blood of Jesus Christ. That takes a lot of belief and faith which is the purpose of the wafer and the symbolism. To outsiders it’s a bit macabre. To the Catholics it’s business as usual in the land of miraculous phenomena.  Biden is, however probably not the best of Catholics, given his long career in politics which took a lot of miracles. Neither side will give in. Joe will still thumb his nose at the doctrine and the church has no choice but to thumb their nose at an ex-American vice President.