[Jeff Chandler]

27 Therefore, whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord.

28 But let a man prove himself: and so let him eat of that bread and drink of the chalice.

BTW, I'm not sure what a "mortal sin" is. What sin can we commit that Jesus did not die to forgive?

Jesus died to forgive all the sins of which we repent. No repentance, no forgiveness.

A mortal sin is a sin so grievous that the sinner is no longer in a state of grace, e.g.: a man commits murder and dies without repentance, he will spend eternity in Hell.

[P-Marlowe]

27 Therefore, whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord.

It still refers to attitude. Unworthily is an adverb and it describes the manner in which you partake of the bread, i.e., your attitude, not your own condition of sinlessness or worthyness.

A mortal sin is a sin so grievous that the sinner is no longer in a state of grace...

Where does it say that in the bible?

[R. Scott]

A mortal sin is a sin so grievous that the sinner is no longer in a state of grace, e.g.: a man commits murder and dies without repentance, he will spend eternity in Hell.

First, I am not Catholic, but I see the difference between mortal and venal sin as one of intent:
The classical definition of mortal sin is given by St. Thomas: "When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object...whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery."
Even more obtuse is the definition given by St. Augustine (Contra Faustum, XXII, xxvii) “as something said, done or desired contrary to the eternal law, or a thought, word, or deed contrary to the eternal law.”
The Catechism of the Catholic Church, paragraph 1857 has “For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."
In modern parlance, if a sin is committed with full knowledge that it is a sin it is mortal. If a sin is committed through blind passion or accident it is venal. Even a lie can be a mortal sin if it is told under oath.
From the Catholic Encyclopedia, “Hence the wickedness of invoking the Divine testimony to confirm an untruth is specially criminal. Prescinding from cases of ignorance or insufficient deliberation this sin is reputed to be always mortal. When in doubt one cannot without perjury swear to a thing as certain. When mental reservation is permissible it is lawful to corroborate one's utterance by an oath, if there be an adequate cause.”
With such a definition we have a whole lot of people living in mortal sin. If the Church’s rules are followed to the letter, there would be few people at the communion rail.