The use of the Old Testament by followers of Jesus
How the Old Testament applies to followers of Jesus is something that confuses a lot of people, whether those people are followers of Jesus or not. "If it's in the Bible, why do you not obey it?" skeptics ask.
The point of the Old Testament law is to show people that they can't obey God using their willpower. That's the reason for the command to be perfect -- to demonstrate that we cannot force ourselves to obey (Leviticus 19:2; Matthew 5:48). The law was specifically intended to be an impossible standard. Because it is an impossible standard, followers of Jesus do not use the Old Testament laws as an outline of requirements for salvation. That would violate the New Testament, which explicitly states, "For by works of the law no human being will be justified... Since through the law comes knowledge of sin" (Romans 3:20) and, "all who rely on works of the law are under a curse" (Galatians 3:10). Reliance on the law rejects Jesus (Galatians 3:13), the only hope of salvation we have. He saves us because of His mercy, not because of anything we do (Titus 3:5).
What is the purpose of the law, and similarly of the whole Old Testament? Paul describes its purpose as twofold: 1) to restrain violations of God's commands, which would be wrong behavior (Galatians 3:19), and 2) to act as a guide to right behavior like a schoolteacher specifically tasked with teaching manners (Galatians 3:24).
Per Scripture, the law increases the human tendency to sin, to the point that we are described as being imprisoned by sin (Galatians 3:23, Romans 7:7-8). However, now that Jesus has come, the law has been fulfilled (Galatians 3:25-26). The Old Testament still has value -- the example of Israel warns us against disobedience, and the prophetic passages predict the coming of Jesus.
Now, Jesus instructs His followers; we follow His commands, not those of Moses. Followers of Jesus must obey those commands. Jesus Himself put it best, "You shall love the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: you shall love your neighbor as yourself. On these two commandments depend all the Law and the Prophets" (Matthew 22:37-40). Obedience to these commands is possible through the Holy Spirit, although Christians often do so imperfectly.