Reading the Bible and applying the Word to our own circumstances is indeed tricky business. What parts should we interpret literally, and what parts are mysterious double entendres? Why does Leviticus 19:27-28 tell us not to trim our sideburns or get tattoos? These are just two examples of how confusing it can quickly get regarding our own personal practice, and to take such examples literally would frankly make me a perpetual sinner.
However, this is not what God is trying to tell us. The truth is, during this time, the Jews learned many superstitious and cult-like customs while in Egypt. The practices often included self-harm, such as making deep gashes to the face and other extremities during times of mourning, and hot ironing their skin to permanently mark themselves in order to proclaim false deities. But God says, "Hey, not necessary!" (author's dialogue, not directly stated in Bible), and continually reiterates in this chapter, "I am the LORD," indicating that numerous false gods and prophets were receiving the glory that really belonged to Him.
As Christians, we may get in the habit of regarding certain Scripture that doesn't seem to individually apply to us as "Old Testament," and therefore inapplicable. To recap, the Old Testament consists of 30 books, and the first five books being what we regard today as the Torah: Genesis, Exodus, Leviticus, Numbers and Deuteronomy. This is the Word of God initially given to the "chosen" land of Israel, not to Christians, which consists of themes that emphasize obedience, ethics and ritual purity so that the Israelite's may know just how to please and worship the Lord. I'm talking really primitive stuff, man. People were all over the place with their worship style, as we just heard, and really needed some direction.
It wasn't until Jesus came along and sacrificed Himself on the Cross for us that He Himself fulfilled the law and ended the need for sacrificial and ceremonial practices (Acts 13:39, Romans 10:4 NRSV). For during Jesus' lifetime, He and His disciples did not practice such strict letters of the law, such as performing ritual hand washings before eating and denying work on the day of the Sabbath, rules the Pharisees simply obeyed and claimed self-righteous from. However, Jesus' intention was not to abolish the moral and ethical principles commanded in the Old Testament. Rather, He affirmed that obedience to the Lord must come from the heart and not from the external motions.
The Beatitudes given by Jesus in Matthew 5 give a raw explanation of the deep and true intentions we must have in our hearts upon following Him. We don't necessarily need to make the same kind of ritualistic sacrifice explained in the Old Testament, but we very well may need to sacrifice the impurities of our hearts for the sake of Jesus Christ.
An analogy between the Old versus New Testament and the national versus state laws creates an adequate comparison: The Old Testament is similar to national law in that it is "universal," almost fundamental, knowledge. These are the laws that apply to any situation in any state. The New Testament is similar to state law in that it holds power, but is ultimately a revised and reformed version of national law that corresponds specifically to a state. It isn't legal in any place to commit murder according to national law, but, if you live in Minnesota and go to Texas to commit the crime, nope, it's still not legal.
The Old Testament holds the Ten Commandments, all (except one) of which are supported in the New Testament, but when the Pharisees in Matthew 22:37-40 ask Jesus which commandment is greatest of all, Jesus replied, "'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments." This seems to be a revised and reformed version of the Old Testament Commandments, one that we must continually pursue and will never be self-justified by--one that we cannot simply externally obey like the Pharisees thought to do.
However, neither law works without use of the other law, as they are interchangeable and dependent to one another. The way in which God handles mankind seems to be ever changing, but His love, purpose and promise remains solidified and faithful. "Jesus Christ is the same yesterday and today and forever." Hebrews 13:8.