3 Nephi 18

After a long day, the Savior chose to remain and teach the people of the Americas the doctrine of the "sacrament" as the Savior -

"...commanded his Disciples that they should bring forth some bread and wine unto him."  (Book of Mormon  3 Nephi 18:1)

And after the people had eaten and drank and were filled He instructed -

"...unto the Disciples: Behold there shall one be ordained among you, and to him will I give power that he shall break bread and bless it and give it unto the people of my church, unto all those who shall believe and be baptized in my name."  (Book of Mormon  3 Nephi 18:5)

In other words, under the direction of priesthood keys, the sacrament could only be administered.  And it is in the sacramet that with  the bread -

"...this shall ye do in remembrance of my body, which I have shown unto you..."  (Book of Mormon
3 Nephi 18:7)   I personally consider this an instruction to remember His mission (including premortal and postmortal), life, physical sufferings and afflictions, and resurrection.  And of the wine or water, those that drink  -   "...shall do it in remembrance of my blood, which I have shed for you..."  (Book of Mormon  3 Nephi 18:11)   And in partaking of these emblems -   "...that ye do always remember me..."  (Book of Mormon  3 Nephi 18:7)   and -   "...that ye are willing to do that which I have commanded you."  (Book of Mormon  3 Nephi 18:10)   Previously, the Savior taught His -   "...doctrine, and whoso buildeth upon this buildeth upon my rock..."  (Book of Mormon  3 Nephi 11:39)   Now He instructs the sacrament is building upon the rock saying -   "...I give unto you a commandment that ye shall do these things. And if ye shall always do these things blessed are ye, for ye are built upon my rock."  (Book of Mormon  3 Nephi 18:12)   The Savior made it clear that -   "...this is the commandment which I give unto you, that ye shall not suffer any one knowingly to partake of my flesh and blood unworthily, when ye shall minister it; 29 For whoso eateth and drinketh my flesh and blood unworthily eateth and drinketh damnation to his soul; therefore if ye know that a man is unworthy to eat and drink of my flesh and blood ye shall forbid him."  (Book of Mormon   3 Nephi 18:28 - 29)

The saints have to be careful not interpreting this to mean having committed any manner of sins.  Only those types of sins that may be judged by priesthood leaders that would consider disciplanary action.  But even so -

"Nevertheless, ye shall not cast him out from among you, but ye shall minister unto him and shall pray for him unto the Father, in my name; and if it so be that he repenteth and is baptized in my name, then shall ye receive him, and shall minister unto him of my flesh and blood."  (Book of Mormon   3 Nephi 18:30)

Why?

"...for unto such shall ye continue to minister; for ye know not but what they will return and repent, and come unto me with full purpose of heart, and I shall heal them; and ye shall be the means of bringing salvation unto them."  (Book of Mormon   3 Nephi 18:32)