You can still follow the old law, but the condition is still the same: everything is applicable. If you fail in one, you have sinned. The stoners were guilty in the moral realm (adultery) if they commited even a minor infraction in the ceremonial! I recommend you read Deuteronomy 28-30.
James 2:10 NET For the one who obeys the whole law but fails in one point has become guilty of all of it.
Background to the Mosaic Law
The Purpose of the Law
The law was given to make Israel God's People, Abraham's Seed, and to bless the world, to bring it back to God, reverse the expulsion from Eden.
Israel needed to:
Obey the law
Impress the surrounding nations
Achievement, success rate
Rahab turned to God on seeing the victories given to Israel.
Why were the achievements so few?
Because the observance of law was so insignificant.
Because the law itself was an inferior covenant
Even if the people depended on God, and took possession of the land, the pagan nations who were impressed and converted to Judaism would find forgiveness, but not empowerment, ability to live the eternal life, chayei olam. Moses quickly realised that this was an inferior glory, but did not want to dishearten the people. He covered his face with a veil so that the people would not be demoralised at seeing the glowing radiance fade from his face. They had just left Egypt, were wanderers in the wilderness and needed all the hope and encouragement they could get. Telling them that full redemption was a future event was not exactly the message they needed to hear.
When Jesus fulfilled the law, He won the rewards associated with Sinaitic Law. God blessed His Nation when Jesus, representing its people, Israel, obeyed the law.
To be pure, the citizen needed to make the right choices with regard to the natural drives of his body: harvesting the fruits of your own land was good, those from your neighbours land, bad. Sex with your wife was good, with your neighbour's wife, bad. The appetites themsrlves, yetzer hara, were themselves not bad, only the wrong exercising of those appetites. Man, made in the image of God, with a moral compass, could differentiate and choose, unlike wild beasts. Hence the concept of justice, as opposed to the law of the jungle, might is right.
Mercy and love are also what characterize the human race. Only humans do not destroy the weak in their family. They lay down their lives (categorised by the various acts of sacrificing one's own reserve food, shelter and resources, to protect the weak) .
In order to be the people and the nation God required, Israel found it had to have a right mind and a right body. The mind had to be subdued, by accepting the word of God, and the body had to be broken, to serve the mind, just as a horse needed to be subdued, to be useful for riding. The right mindset in the right body could create the right people in the right nation.
Joshua could not give Israel closure, only God could. The virgin birth proved that. Just as Isaac was possible because of an ovum provided by God (Sarah's womb was as good as dead) and his birth proof of God's causation, Jesus was proof that it was God who provided the Lamb, from a spermatozoon supernaturaly produced and implanted.
The blood of Jesus could do what no blood of bulls and goats did. It cleansed Israel and it cleansed her Temple. It cleansed the mind of those who agreed with the Gospel (here was the Messiah who vindicated the righteous and reconciled them with God, and who could redeem them, both their minds and their bodies, through confession : con>with, fess>speech, agreement) . If you were righteous, you could be vindicated; if you were unrighteous, you could repent and then be reconciled.
Now, the Church is to be a pure people in a pure community, its members to have a pure mind in a pure body. An obedient church could now act out that righteousness:
Romans 7:24-25 NET Wretched man that I am! Who will rescue me from this body of death? Thanks be to God through Jesus Christ our Lord! So then, I myself serve the law of God with my mind, but with my flesh I serve the law of sin.
Are there some cases where it is sinful or forbidden to follow any of the laws in the Old Testament, now that Jesus has fulfilled these laws (Romans 10:4; Galatians 3:23-25; Ephesians 2:15)?
Now that the church enjoys the blessings meant for God's People, what remains of the law? Since Jesus won redemption for the church through obedience to the law, obedience to the law now wins redemption for the world:
Colossians 1:24 NET Now I rejoice in my sufferings for you, and I fill up in my physical body – for the sake of his body, the church – what is lacking in the sufferings of Christ.
The form in which you follow law could be the Mosaic law, which requires ALL requirements to be fulfilled, or you could follow the law of liberty and love, the Royal Law, requiring holy living, enabling God to use one for ministry.
2 Timothy 2:20-21 NET Now in a wealthy home there are not only gold and silver vessels, but also ones made of wood and of clay, and some are for honorable use, but others for ignoble use. So if someone cleanses himself of such behavior, he will be a vessel for honorable use, set apart, useful for the Master, prepared for every good work.
Following the Mosaic Law is not sin. Its just impossible, since ALL requirements cannot be met, leading to death, impotency. No benefit is achieved. Mosaic Law observed through faith provides only guardianship care, which is already a obsolete, redundant in the Gospel, which provides restoration and empowerment of the prodigal. However, however, it is better to misunderstand the Gospel in seeing the continued necessity of Mosaic Law as a lifestyle, not as salvific, than to misunderstand the Gospel by viewing the requirement of the new, Royal Law to be mental assent to the message that Jesus is the Messiah, the Holy One of God (Free Grace Theology) leading to the gift of immortality.
Would the Jews have been sinners in obeying Law in stoning the adulteress?
Talmudic interpretation makes a right call in its interpretation of Torah. There, even lex talionis, eye for an eye, calls for monetary compensation for accidental bodily injury of another person, and even in that, mercy could be shown to those unable to pay.
It boils down to interpretation:
Mark 2:27 NET Then he said to them, “The Sabbath was made for people, not people for the Sabbath.
How is the law made for man, and not man for the law?
It was supposed to give life!
Romans 7:10 NET and I died. So I found that the very commandment that was intended to bring life brought death!
Hillel "Whatever is hateful to you, do not do unto another. That is the whole of the Torah. The rest is commentary."