The OT contains many important foreshadows that teach us about God's nature and His plan of redemption. The light of Christ brings full substance to these foreshadows so that we can fully see what God was teaching us through them, which make them all the more important to continue to observe in remembrance of him. For example, in 1 Corinthians 5:6-8, Paul spoke in regard to how Passover foreshadowed Christ by drawing the connection of him being our Passover Lamb, however, instead of concluding that we no longer need to bother keeping Passover, he concluded that we should therefore continue to keep it. The foreshadows testify about Christ and as his followers should live in a way that testifies about him by observing them.
While we are under the New Covenant and not the Mosaic Covenant, we are nevertheless still under the same God with the same nature and therefore the same laws for how to testify about His nature. For example, the way to testify about God's righteousness is straightforwardly based on God's righteousness, not on any particular covenant, and God's righteousness is eternal, so any instructions that God has ever given for how to testify about His righteousness are eternally valid regardless of which covenant someone is under. It's not mixing covenants when they are all made with the same God with the same eternal nature.
Jesus began his ministry with the Gospel message to repent for the Kingdom of God is at hand (Matthew 4:17-23), and the Mosaic Law was how his audience knew what sin is, so repenting from our disobedience to it is an integral part of the Gospel of the Kingdom, which Jesus prophesied would be proclaimed to the nations (Matthew 24:12-14). The same goes for how Peter's audience knew what sin is when he told them to repent for the forgiveness of sin in Acts 2:38. In Romans 15:4, Paul said OT writings are for our instruction and in 15:18-19, his Gospel message involved bringing the Gentiles to obedience in word and in deed, so his Gospel was on the same page in regard to teaching repentance from our sins.
In Psalms 119:29-30, David wanted to put false ways far from him, for God to be gracious to him by teaching him to obey His law, and to choose the way of faithfulness, so this has always been the way of salvation by grace through faith. In Romans 1:5, we have received grace in order to bring about the obedience of faith. In Titus 2:11-14, our salvation is described as being trained by grace to do what is godly, righteous, and good, and to renounce doing what is ungodly, so God graciously teaching us to obey His laws for how to do that is itself the content of His gift of salvation. Our salvation is from sin and sin is the transgression of God's law (1 John 3:4), so being trained by grace to live in obedience to God's law through faith is what it means to receive the gift of Jesus saving us from living in transgression of God's law. God's law commands to keep the Sabbath holy, so being saved from living in transgression of the Sabbath is part of the content of our salvation from sin, though we do not earn our salvation by keeping the Sabbath holy.
The topic of Romans 14 stated in the first verse in regard to how to handle disputable matters of opinion, not in regard to whether followers of God should follow God, so nothing in the chapter should be interpreted as promoting rebellion against what God has commanded. Where God has given no command, then we are free to follow our own opinions, but where God has given as command, then human opinion must yield. In Romans 14:4-5, it speaks about eating or refraining from eating unto the Lord, so it is speaking about those who esteem certain days for fasting as a disputable matter of opinion. God gave no command to fast twice a week, but that had become a common practice in the 1st century, and those who were choose to fast as a matter of opinion were passing judgement on those who were not, and were in trunk being resented, so it was exactly this sort of judging each other over opinions that Paul was seeking to quell in this chapter. On the other hand, God has commanded to keep the 7th day holy, so whether we choose to do that is not a disputable matter of opinion, but a matter of obedience to God. Paul was not suggesting that we are free to commit theft, adultery, murder, idolatry, break the Sabbath, or disobey any of God's other commands just as long as we are convinced in our own minds that rebellion against what God has commanded is ok, but rather that was only said in regard to things that are disputable matters of opinion.
In 1 Peter 1:16, we are told to have a holy conduct for God is holy, which is a quote from Leviticus where God was giving instructions for how to have a holy conduct, which includes keeping God's Sabbaths holy (Leviticus 19:2-3), so following those instructions is testifying about God's holiness, while refusing to follow them is bearing false witness against God's holiness. In 1 Peter 2:9-10, Gentiles are now included as part of God's chosen people, a holy nation, a royal priesthood, and a treasure of God's own possession, which are terms used to describe Israel (Deuteronomy 7:6), so Gentiles also have the delight of getting to obey the laws that God gave for how to fulfill those roles. It is contradictory for someone to what to become part of a holy nation while wanting nothing to do with following God's instructions for how to live as part of a holy nation, and it is contradictory for someone who thinks that they aren't under the Law of Moses to think that they need salvation from sin, which is the transgression of it.
