In the Bible, these "rules" were written in the form of the Law. The Law, in our modern Christian culture, seems like an outdated and even slightly cynical concept. I mean, Christ came to save us from legalism, right? True. But if we don't view the Law in the right perspective, we'll miss God's timeless plan. The Law did (and still does) have a purpose. God didn't create something just because He wanted to free us from it. He wanted to free us from sin, and the Law was simply a part of His plan to do so.
Our perspective of the Law starts all the way at the beginning. Just after God creates humanity, He tells them, "You shall not". He establishes obedience by... guess what? A rule. Had He not given them this commandment, how would they know what was wrong and what was right? This was the role of the Law. Romans 5:13 says "for sin indeed was in the world before the law was given, but sin is not counted where there is no law." Before the law, nothing was officially established as right or wrong. The King James Version explains it like this: "Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith." (Galatians 3:24, emphasis added). The Law was our teacher. It taught us right from wrong. Because before God could free us from sin, we had to know what it was we were being freed from! If we hadn't had the law, Christ's death would have been in vain, because we wouldn't have known the reason.
Now that we see the relevance of the Law, what is its relevance today? Since we're freed from the Law, we don't need to follow it, right? So we don't have to worry about getting tattoos (Leviticus 19:28) or sacrificing lambs (Leviticus 4, 23, Numbers 7, etc)? Not exactly. You see, God's law wasn't just a list of do's and don't's. It was His will. The Law showed the deepest desires of the Lord: to be glorified, to be worshiped and for us to sacrifice to Him. God's desires have never changed. We aren't required to keep the law because Christ has set us free. We live in grace. But still, the concepts of the law show what God desires. You can do what you like, but God has already given us His will. As 1 Corinthians 10:23 says, "'All things are lawful,' but not all things are helpful. 'All things are lawful,' but not all things build up."
Because of God's grace, we can always be forgiven. But that does not mean we should live in sin. "What shall we say then? Are we to continue in sin that grace may abound? By no means! How can we who died to sin still live in it?" (Romans 6:1-2). Live in the grace of Christ, but keep His Word ever close to your heart.
Next time you talk to God, thank Him for the Law. Thank Him for showing us what sin is. But thank Him all the more for His saving grace. Thank Him that we are no longer held to the Law; that He accepts us just as we are. And thank Him for His awe-inspiring plan of salvation.