If you sin knowingly, you are ineligible for salvation.
And remember also the words which Amulek spake unto Zeezrom, in the city of Ammonihah; for he said unto him that the Lord surely should come to redeem his people, but that he should not come to redeem them in their sins, but to redeem them from their sins. (Helaman 5:10)
For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins.... (Hebrews 10:26)
9 Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind,
10 Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. (1 Corinthians 6)
But remember that he that persists in his own carnal nature, and goes on in the ways of sin and rebellion against God, remaineth in his fallen state and the devil hath all power over him. Therefore, he is as though there was no redemption made, being an enemy to God; and also is the devil an enemy to God. (Mosiah 16:5)
But wo, wo unto him who knoweth that he rebelleth against God! For salvation cometh to none such except it be through repentance and faith on the Lord Jesus Christ. (Mosiah 3:12)
Therefore God gave unto them commandments, after having made known unto them the plan of redemption, that they should not do evil...for on such the plan of redemption could have no power, for the works of justice could not be destroyed, according to the supreme goodness of God. (Alma 12:32)
If, along the way, you sin—either by disobeying something you previously knew or resisting or rejecting something newly revealed— you remove yourself from his promises, and you must repent and renew your dedication to the Lord.
Two changes happen when you willingly disobey even one commandment.
First, you immediately recover the penalty of your past sins. God’s law requires you to obey all of the commandments you know. If you break even one commandment, you are held accountable as one who does not obey his law:
For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all. (James 2:10)
If you break even one commandment, you are responsible before God for the penalty of all your sins, including those which were previously forgiven:
And now, verily I say unto you, I, the Lord, will not lay any sin to your charge; go your ways and sin no more; but unto that soul who sinneth shall the former sins return, saith the Lord your God. (D&C 82:7)
Second, when you willingly break even one commandment, you lose some of your capacity to discern between right and wrong and regress to a lower level of understanding, making it impossible for you to see and choose as great of outcomes as you could previously.
Those who willingly disobey God will decrease in understanding, making it harder to see paths to improvement and to obtain what they lead to:
And he that repents not, from him shall be taken even the light which he has received; for my Spirit shall not always strive with man, saith the Lord of Hosts. (D&C 1:33)
You should always obey all of the commandments from God which you know.