“And now behold, this is the commandment which I give unto you, that ye shall not suffer anyone knowingly to partake of my flesh and blood unworthily when ye shall minister it. 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. 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.” (LDS 3 Nephi 18:28-30)
“See that ye are not baptized unworthily. See that ye partake not of the sacrament of Christ unworthily, but see that ye do all things in worthiness, and do it in the name of Jesus Christ, the Son of the living God.” (LDS Mormon 9:29)
One of the purposes of baptism and the sacrament is to establish a baseline commandment for you to keep. A person can come, confessing their sins and desiring to be reconciled to God. Christ essentially says to them:
Repent of your sins and keep my commandments. Now, this if my first commandment to you: be baptized by water for the remission of your sins. Yea, go down into the water and be immersed in my name and come forth again out of the water. If you do this as I have instructed you, then you shall be visited with fire and with the holy ghost.
And regarding the sacrament, in partaking of bread and wine in remembrance of him:
Those who are baptized in my name and come unto me repenting, yea confessing your sins and forsaking them and desire to show that you will keep my commandments, behold this is my commandment to you: Partake of my flesh and blood by breaking bread and blessing it in my name and by drinking wine that is also blessed in my name. This shall you do that you may show the Father that you indeed do keep my commandments which I give you and if you keep my commandments which I give you, you shall always have my spirit to be with you. But if you do not the things which I say and command you, after you have heard my voice and know my commandments for you but will not keep them, it would have been better for you if you had not heard my voice and known me. And whoever does these things but not as I have instructed them, the same does eat and drink damnation to their souls, for they do these things not according to my words but they show the Father that they do rebel against me. Yea, it would have been better for them if they had not known me. They have heard my voice but will not do as I have said but do eat and drink damnation to their souls.