We need to believe the truth and use the truth a guide for how to live our lives, however, the truth is not arbitrary, but rather God's nature is truth and part of the content of His gift is being trained by grace to live in a way that testifies about the truth. The Mosaic Law is truth because it is God's instructions for how to testify about His nature (Psalms 119:142), and Jesus is the exact expression of God's nature (Hebrews 1:3), which he testified about through setting a sinless example of how to do good works in obedience to the Mosaic Law, so Jesus is the truth made flesh (John 14:6-11). The Mosaic Law is God's instructions for equipping us to do every good work (2 Timothy 3:16-17), so partaking in God's nature through doing good works in obedience to it is what it looks like to believe the truth and to accept God's gift, while disobedience to the Mosaic Law is what unbelief and sin look like (1 John 3:4).
What we believe is expressed through our actions, which is why James 2:17-18 says that faith without works is dead and that he would show his faith by his works, so doing good works in obedience to God is what faith looks like, which is also why Jesus said in Matthew 23:23 that faith is one of the weightier matters of the Mosaic Law, and why every example of faith listed in Hebrews 11 is also an example of works. In Romans 6:19-23, no longer presenting ourselves as slaves to impurity, lawlessness, and sin is contrasted with now presenting ourselves as slaves to God and to righteousness leading to sanctification, and the goal of sanctification is eternal life in Christ, which is the gift of God, so obedience to the Mosaic Law is what it looks like to accept God's gift of salvation. In Psalms 119:29, David wanted God to be gracious to him by teaching him to obey the Mosaic Law, in Romans 1:5, we have received grace to bring about the obedience of faith, and 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, which is what the Mosaic Law was given to instruct how to do, so God graciously teaching us to obey it is again itself part of the content of God's gift of salvation. Our salvation is from sin and sin is the transgression of the Mosaic Law, so being trained by grace to live in obedience to it through faith is what it looks like to accept the gift of Jesus saving us from living in transgression of it.
In Ezekiel 36:26-27, the Spirit has the role of leading us to obey the Mosaic Law, in John 16:13, the Spirit has the role of leading us in truth, and again the Mosaic Law is truth. In John 8:31-36, it is sin in transgression of the Mosaic Law that puts us in bondage while it is the truth that sets us free. In John 17:17, God’s word is truth, and the Mosaic Law is God’s word (Deuteronomy 5:31-33). In 2 Timothy 3:8, those who oppose the truth also oppose Moses, being of corrupted minds, and disqualified in regard to the faith. In Romans 8:4-7, those who walk in the Spirit are contrasted with those who have minds set on the flesh who refuse to submit to the Mosaic Law. In Galatians 5:19-22, everything listed as works of the flesh that are against the Spirit are also against the Mosaic Law, while all of the fruits of the Spirit are aspects of God's nature that are in accordance with it.