Our salvation is from sin (Matthew 1:21) and sin is the transgression of God's law (1 John 3:4), so living in obedience to God's law is not something external to salvation that we need to do first in order to have it, or that maintains it, or that is evidence of it, but rather living in obedience to it is intrinsically the content of what God's gift of salvation is. For example, honoring our parents is intrinsically the content of God's gift of saving us from not honoring our parents. In Psalms 119:29-30, he wanted to put false ways far from him, for God to be gracious to him by teaching him to obey His law, and he chose the way of faith by setting it before him, so this has always been the one and only way of salvation by grace through faith.
The way that we choose to live expresses what we believe. In other words, our goal is to be in God's image, so if we believe that God is good, then we will do good, if we believe that God is just, then we will do what is just, and so forth. God is trustworthy, therefore His law is also trustworthy (Psalms 19:7), so the way to trust God is by obediently trusting in God's instructions, and it is contradictory to think that we just need to trust God instead of trusting in His instructions. In Proverbs 3:5-7, we have a choice between whether we are going to lean on our own understanding of right and wrong by doing what is right in our own eyes or whether we are going to trust God with all of our heart to correctly divide between right and wrong by obeying His instructions in all of our ways and He will make our way straight.
This is why the Bible repeatedly connects our belief/faith in God with our obedience to Him. For example, in Matthew 23:23, Jesus said that faith is one of the weightier matters of the law. In James 2:18, he would show his faith by his works. In Romans 1:5, we have received grace in order to bring about the obedience of faith. In Romans 3:31, our faith upholds God's law. In Revelation 14:12, those who kept faith in Jesus are the same as those who kept God's commandments. In John 3:36, believing in Jesus is equated with obeying him. In Hebrews 11, every example of faith is an example of works. In Numbers 5:6, disobedience to God's law is described as breaking faith. In Hebrews 3:18-19, unbelief is equated with disobedience. And so forth, so the Bible never treats belief as being an alternative to obedience to God.
Nowhere does the Bible say that.
Sin is what is contrary to God's nature, so the fact that God's nature is eternal, means that what is contrary to God's nature is also eternal. When people commit sin is a different issue than what sin is.