For the priesthood being changed, there is made of necessity a change also of the law.  (Heb 7:12)

With the Resurrection of Jesus, Jesus the Christ became our new heavenly high priest.  Jesus Christ is the mediator of the New Covenant, which replaces the Old Covenant.  [Note:  Also, see Old & New Covenants: Structure & Terminology.]

Goals: (spending eternity in heaven)

He that overcometh shall inherit all things; and I [God] will be his God, and he shall be my son.  (Rev 21:7)

Blessed are they that do his [God's] commandments, that they may have right to the tree of life, and may enter in through the gates into the city.  (Rev 22:14)

... what good thing shall I do, that I may have eternal life?
 
Jesus: ... if thou wilt enter into life, keep the commandments.  (Mat 19:16-17, Mar 10:17-19, Luk 18:18-20)
 
Jesus:  If ye love me, keep my commandments.  (Joh 14:15)

Jesus:  He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him.  (Joh 14:21)

Jesus:  If ye keep my commandments [under the new covenant], ye shall abide in my love; even as I have kept my Father's [God's] commandments [under the old covenant], and abide in his love.  (Joh 15:10)

The second is like unto it,  Thou shalt love thy neighbor as thyself.  (Mat 22:39, Mat 19:19, Mar 12:31,33, Luk 10:27, Rom 13:9, Gal 5:14, Lev 19:18, Mat 5:43)
On these two commandments [above] hang all the law and the prophets.  (Mat 22:40)

Thou shalt do no murder  (Mat 19:18)  Do not kill  (Mar 10:19, Luk 18:20)

Thou shalt not commit adultery  (Mat 19:18, Mar 10:19, Luk 18:20)

Thou shalt not steal  (Mat 19:18, Mar 10:19, Luk 18:20)

Thou shalt not bear false witness  (Mat 19:18, Luk 18:20)  Defraud not  (Mar 10:19)

Honour thy father and thy mother  (Mat 19:19, Mar 10:19, Luk 18:20)

Thou shalt not covet  (Rom 13:9)  [ This commandment is not mentioned by Jesus here (because it is likely included in the commandment to "love thy neighbor as thyself"), but is mentioned in Romans and in many other places.]

This is my commandment, That ye love one another, as I have loved you.

Greater love hath no man than this, that a man lay down his life for his friends.

Ye are my friends, if ye do whatsoever I command you.  (Joh 15:12-14)

These things I command you, that ye love one another.  (Joh 15:17)

Now therefore why tempt ye God, to put a yoke upon the neck of the disciples [Gentiles who become Christians.  Also?, Jews who become Christians.], which neither our fathers [Jews] nor we [Jews, before becoming Christians] were able to bear?  But we believe that through the grace of the Lord Jesus Christ we shall be saved, even [just] as they [our fathers] [are saved].  (Act 15:10-11)

For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things;
 

That ye abstain
 
from meats offered [sacrificed] to idols, and

from blood, and

from things strangled, and

from fornication [sexual immorality]:

 
from which if ye keep yourselves, ye shall do well.  Fare ye well.  (Act 15:28-29)

The Old Testament (OT), Old Covenant, law defines sin ("what sin is" and "what is sin").

The definition of sin under the OT & NT remains unchanged:
 
Think not that I [Jesus] am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.  (Mat 5:17)

For verily I [Jesus] say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.  (Mat 5:18)

Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.  (Mat 5:19)

 
What happened to the Old Covenant?  In that he [God] saith, A new covenant, he [God] hath made the first [covenant] old.  Now that which decayeth and waxeth old is ready to vanish away.  (Heb 8:13)

Keeping the commandments and the law are about the condition of our hearts:
 
about loving God, our neighbor, and ourselves.

When we fail to love God, our neighbor, or ourselves,
 

we sin first in our hearts,  (see Mat 5:21-48)
 
and then in our actions.

He that overcometh shall inherit all things; and I [God] will be his God, and he shall be my son.  (Rev 21:7)

Blessed are they that do his [God's] commandments, that they may have right to the tree of life, and may enter in through the gates into the city.  (Rev 22:14)