Many on this board understand that Christians are not under the 10 Commandments or the Law. That is completely distinct from "no longer believing" in them.
Most of the commandments are reiterated in the New Testament, which makes clear those actions are still sinful. The difference is that the Law can only itemize a limited number of actions, while the Holy Spirit can speak to the Christian about any action.
The Holy Spirit speaks to the Christian internally, bringing conviction of sin. He also speaks through a variety of other means, including other people. So, to answer your opening question directly, sin is what the Holy Spirit says it is.
“Now we know that
what things soever the law saith, it saith to them who are under the law:
that every mouth may be stopped, and all the world may become guilty before God.
Therefore
by the deeds of the law there shall no flesh be justified in his sight: for
by the law is the knowledge of sin.”
Romans 3:19-20
when you learn the law of Moses your learning what sin is it’s consequence is death in other words to serve sin is a death sentance
“Know ye not, that
to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey;
whether of sin unto death, or of obedience unto righteousness? For when ye were the servants of sin, ye were free from righteousness. What fruit had ye then in those things whereof ye are now ashamed?
for the end of those things is death. For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.”
Romans 6:16, 20-21, 23 KJV
“Whosoever committeth sin transgresseth also the law:
for sin is the transgression of the law.”
1 John 3:4
Sin is to transgress what the law of Moses said and it’s consequence is death penalty some details matter like the is one
“Therefore
to him that knoweth to do good,
and doeth it not, to him it is sin.”
James 4:17
if I know it’s wrong to lie which the law of Moses declares and I lie Ive sinned if I’ve done something wrong but didn’t know or don’t know it’s wrong I havent sinned I’m just ignorant of sin. So another aspect of what constitutes sin is I have to be aware of what I’m doing being wrong and doing it anyways or knowing The right thing to do and refusing to do it if I knowingly violate my faith my understanding of the gospel I’ve sinned and need to repent
I like your post there being a Christian doesn’t mean now we get to break the commandments it means we know can see and hear God clearly rather than peering through a veil not actually hearing his word like the law so for instance the commandments are here in the New Testament
“Owe no man any thing, but to
love one another: for he that loveth another hath fulfilled the law.
For this,
Thou shalt not commit adultery,
Thou shalt not kill,
Thou shalt not steal,
Thou shalt not bear false witness,
Thou shalt not covet; and if there be any other commandment, it is
briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself. Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.”
Romans 13:8-10 KJV
Saying I love everyone won’t help but treating people with love is the thing that fulfills the law if I steal I’m stealing from a man I’m doing evil to my fellow
Man this isnt love I need to repent
if I kill or do violence against another I’m not loving them . If I cheat on my wife I’m not loving her , if I covet my neighbors life and possessions I’m not loving them ect the law was always about
what not to do don’t lie , don’t cheat , don’t steal from each other don’t hurt each other or kill each other ect
but notice paul doesnt say if you keep the commandments now you have fulfilled love
because the law telling me not to kill another doesn’t mean I love them if I don’t kill them love is more than tHat love is what helps others it acts to benefit others it’s like the Good Samaritan it helps when it is able and another has need
the gospel is about what God really told man to do and be and the le is about what God told sinners not to continue doing or they would die
knowing sin can’t save us but knowing righteousness does Old Testament and new
“
The law and the prophets were
until John:
since that time the kingdom of God is preached, and every man presseth into it.”
Luke 16:16 KJV
“Therefore
by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference:”
Romans 3:20-22 KJV
believing the gospel will save us the New Testament but not this
“and not as
Moses, which put a vail over his face, that the children of Israel could not stedfastly look to the end of that which is abolished: but
their minds were blinded: for until this day remaineth the same vail untaken away in the reading of the old testament; which vail is done away in Christ. But even unto this day, when Moses is read, the vail is upon their heart.”
2 Corinthians 3:13-15 KJV
The Old Testament is in the at position and perspective but this is the gospel when the veil is removed
“But we all, with open face
beholding as in a glass the glory of the Lord, are
changed into the same image from glory to glory, even as by the Spirit of the Lord.”
2 Corinthians 3:18 KJV