Christianity is the faith and life of being a disciple of Jesus Christ. A Christian is not a disciple of Moses!
Before the Law of Moses, men would be obeying by instinct the commands of Law, as is described by Paul:
"For when Gentiles who have no Law obey by natural instinct the commands of the Law, they, without having a Law, are a Law to themselves; since they exhibit proof that a knowledge of the conduct which the Law requires is engraven on their hearts, while their consciences also bear witness to the Law, and their thoughts, as if in mutual discussion, accuse them or perhaps maintain their innocence—" (Rom 2:14-15, Weymouth)
Sin existed in the world before the law because of the sense of right and wrong was written on their hearts, therefore people then died; yet they did not sin in violating the like command given to Adam:
"Therefore, as through one man sin entered into the world, and death through sin; and so death passed unto all men, for that all sinned:— for until the law sin was in the world; but sin is not imputed when there is no law. Nevertheless death reigned from Adam until Moses, even over them that had not sinned after the likeness of Adam's transgression, who is a figure of him that was to come." (Rom 5:12-14, ASV)
The Old Covenant, the Law of Moses was given to Israel alone in a specific time frame and location:
"And what great nation is there, that hath statutes and judgments so righteous as all this law, which I set before you this day?" (Deut 4:8, ERV)
"These are the commandments, which the LORD commanded Moses for the children of Israel in mount Sinai." (Lev 27:34, ERV)
Our justification is based on "the promise of the Spirit through faith" and not annulled by the Law:
"Now to Abraham were the promises spoken, and to his seed. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ. Now this I say; A covenant confirmed beforehand by God, the law, which came four hundred and thirty years after, doth not disannul, so as to make the promise of none effect." (Gal 3:16-17, ERV)
Moses prophesied that there was a prophet to come to whom they were to listen, who is Jesus:
"Moses indeed said, A prophet shall the Lord God raise up unto you from among your brethren, like unto me; to him shall ye hearken in all things whatsoever he shall speak unto you. And it shall be, that every soul, which shall not hearken to that prophet, shall be utterly destroyed from among the people." (Acts 3:22-23, ERV)
Gentile Christians, the disciples, were not put under the Law of Moses:
"Now therefore why tempt ye God, that ye should put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear? But we believe that we shall be saved through the grace of the Lord Jesus, in like manner as they." (Acts 15:10-11, ERV)
Jesus said in Matthew 5:17-18 that he would fulfil, complete and accomplish all the Law and he did so at the cross:
"When Jesus had received the wine, he said, 'It is finished.' Then he bowed his head and gave up his spirit." (John 19:30, NRSV)
"he has cancelled the bond which was outstanding against us with its legal demands; he has set it aside, nailing it to the cross." (Col 2:14, REB)
"For he is our peace, who hath made both one, and hath broken down the middle wall of partition between us; Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace;" (Eph 2:14-15, KJV)
"In speaking of “a new covenant,” he has made the first one obsolete. And what is obsolete and growing old will soon disappear." (Heb 8:13, NRSV)
*we know the first Covenant included the 10 Commandments (words)
"He was there with the LORD forty days and forty nights; he neither ate bread nor drank water. And he wrote on the tablets the words of the covenant, the ten commandments." (Exod 34:28, NRSV)
"But even if we or an angel from heaven should proclaim to you a gospel contrary to what we proclaimed to you, let that one be accursed! As we have said before, so now I repeat, if anyone proclaims to you a gospel contrary to what you received, let that one be accursed!" (Gal 1:8-9, NRSV)