So what is included in the OT law that Christians are required to follow and observe and what isn't.
The biggest area of disagreement is in regard to following ceremonial laws, such as keeping God's Sabbath, keeping God's other holy days, and keeping kosher. As I pointed out with 1 Peter 1:13-16 and 1 Peter 2:9-10, we are told to have a holy conduct, and the ceremonial laws are God's instructions for how to have a holy conduct. The phrase "to be holy for God is holy" is a reference to Leviticus where God was giving those instructions, such as Leviticus 11:44-45, where refraining from eating unclean animals is part of what it means to have a holy conduct. It is also relatively straightforward that having a holy conduct involves keeping God's holy days. Furthermore, it is not about act like Jews, about about acting according to the holiness of our God.
What I'm trying to say is that the Old Covenant commanded things like circumsizing your children
In regard to understanding circumcision, it is critically important to make the distinction between what God required and what man required. For example, in Acts 15:1, the Pharisees were wanting to required all Gentiles to become circumcised in order to become saved. However, nowhere does God's Law require this, and furthermore God did not even require Jews to become circumcised in order to become saved, though He did require Jews to become circumcised as a sign of the Abrahamic covenant. Likewise, the law mentioned in Acts 10:28 that forbade Jews from visiting or associating with Gentiles is not found anywhere in God's Law, so it is therefore also a man-made requirement, which means that things being discussed in Acts 15:1-11 were in regard to following man-made traditions, though it was nevertheless how the Pharisees thought that the Mosaic Law should be obeyed. So by rejecting this man-made requirement, the Jerusalem Council was upholding God's Law. Furthermore, the Jerusalem Council did not had more authority than God, so they could not countermand any of His commands and it would have been a sin in violation of Deuteronomy 4:2 for them to tell Gentiles not to obey any of them. We must obey God rather than man, so if you nevertheless think that God required all Gentiles to become circumcised in order to become saved and that the Jerusalem Council ruled against obeying what God commanded, then you should follow God instead of them, but again, they never spoke against anyone obeying any of God's commands.
and sacrificing animals for sin atonement, and having a levitical priesthood to attend priestly elements. Are these things not done away with?
In Acts 18:18, Paul took a Nazarite Vow, which involves making offerings (Numbers 6) and in Acts 21:20-24, Paul was on his way to pay for the offerings of others who had taken a similar vow in order to disprove false rumors that he was teaching against obeying the Law and to show that he continued to live in obedience to it. So offerings did not stop with the death and resurrection of Jesus, but only stopped because of the destruction of the temple. However, the Bible prophecies a time when a third and fourth temple will be built and when offerings will resume (Ezekiel 40-46).
And if they truly are not then a good chunk of the new testament is actually a lie.
Again, there is a theme throughout the Bible that we must obey God rather than man, so we must be careful not to take something that was only against obeying the laws of men as being against obeying the laws of the God that we serve, and when we are careful to do this, then we will see that the Bible does not speak against anyone obeying any of God's commands. For example, I have commonly seen Galatians 4:9-11 used to argue against keeping God's holy days, but if you read verse 8, you will see that Paul was addressing these verses to those who were former pagans. This means that they were not formerly obeying God's holy days and therefore could not be returning to keeping them. Paul likewise would not have referred to the holy, righteous, and good Law of God as weak and miserable principles of the world, but rather than is how he referred to man-made teachings, so whatever Paul was referring to them returning to in verse 10 is in the context of paganism, not in regard to obeying God's Law. In Galatians 2:11-14, we have another example where people commonly use to show that Peter was not eating kosher, but what they happened to be eating was never even mentioned as was not even relevant to what was happening. Peter was simply obeying the man-made law mentioned in Acts 10:28 when he stopped visiting or associating with Gentiles, so the way that we have been taught to interpret the NT has systematically taken what was only against obeying man's laws and twisted it to being against obeying the God that we serve. God's Law was given for our own good as instructions for how to live in a way where we will prosper and be blessed (Deuteronomy 6:24), which are not too difficult for us (Deuteronomy 30:11-14, Romans 10:5-10), but instead of considering it by faith to be a delight as it was intended (Romans 7:22), we have consider it to be a heavy burden that no one could bear.
And if they are done away with, then how exactly do we know what has changed and what hasn't.
If taking a particular action was acting in accordance with God's righteousness before Christ came, but now that he has come that is no longer the case, or vice versa, then God's righteousness has changed, but God's righteousness is eternal and does not change, and the same can be said for the way to act in accordance with God's other attributes. All of God's righteous laws are eternal (Psalms 119:160), so it is not the case that God's laws have changed, but it is the case that the circumstances under which they apply have changed. For example, laws in regard to temple practice can only apply when there is a temple in which to practice them. Other laws were meant to instruct how Israel as a nation was to be governed, so things can become tricky in regard to how we should live when we are citizens of a different nation that has its own laws. So understanding how God's Law applies to us today is a matter of careful study, prayer, and guidance of the Holy Spirit.
