The way to believe in God's nature is by seeing the laws that He has instructed and the way to testify about His nature is by obeying them. For example, the way to believe that God is good is by seeing that He has commanded good laws and the way to testify about God's goodness is by doing good works in obedience to them, which is why our good works bring glory to Him (Matthew 5:16). On their other hand, if God had instead commanded things like to commit murder and adultery, then it would be incorrect to believe in the goodness of God. The same is true about other aspects of God's nature, soo the set of laws that the God of Israel chose to give paints us a profile picture of His identity and if someone is following a different set of laws, then the God that they believe in does not have the same identify as the God of Israel. For example, in 1 Peter 1:16, we are told to be holy as God is holy, which is a quote form Leviticus where God was giving instructions for how to do that, so by following those instructions we are testifying about God's holiness and if someone refuses to follow those laws, then they are testifying that the God that they follow is not holy and therefore does not have the same identity as the God of Israel. As representatives of the God of Israel we should live in a way that testifies about His nature by obeying the Mosaic Law.