When the first humans were in Paradise, God told them (Adam and Eve) they could eat from any tree but one, and if they did they would die.
And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat: But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die. (Genesis 2:16-17 KJV)
The serpent came and told Eve they would not die if they ate from this tree. Eve decided to believe the serpent's lie rather than God. She did not trust God, and did not do His will but thought she could be like God:
And the serpent said unto the woman, Ye shall not surely die: For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil. (Genesis 3:4-5 KJV)
Eve ate from the tree of the knowledge of good and evil, and she also seduced her husband Adam to do the same. When God found out, they felt ashamed and noticed they were naked, something they were not even aware of before this. God kicked them out of Paradise, and from this time on this world has been a fallen world, a world where sin has distorted almost everything.
Humans kept on sinning, so God gave them the law in the written form to Moses, which we still know as the Ten Commandments (see Exodus 20:1-17) to make sure what He meant.
The humans still kept on sinning, which is falling away from God and His will, being in rebellion to what He says (His Word). This is called sin.
The law that God gave to the children of Israel after leaving slavery in Egypt to go to the Promised Land is a holy matter, and as such it is eternal. Sin is not the breaking of the law itself, but it is the motivation behind this action, namely to believe we can decide about what is good or evil without asking God about it. This is important: God is holy, and this means in His light any kind of sin is destroyed! Since no human is sinless, we would be destroyed in His light and glory.
We would all be very lost forever, because no human is perfect and can keep the law, and the penalty for sin is death as God proclaimed in Paradise. We would all be lost if there was not God's unconditional love for us humans. He wants to live with us again, just as it was like in Paradise when Adam and Eve could talk with Him there face to face. In His endless love God had a plan…
He sent Jesus, His Son, who was blameless, to earth as a human to pay the debt for our sins in our place on the tree. If we believe in this, God will forgive us our shortcoming, our sin, our distrust in Him, our listening to the devil - and we will have eternal life with Him as it was planned in the beginning when God made us after His own image asnd a free will, even if this meant we could turn against Him and listen to the devil and trust him more than God. But there is no love without freedom, is there?
This last part, from when Jesus came into this world, can be found in the New Testament, which is fundamental to the Christian faith. The Jewish faith was before this, and is also fundamental to the Christian faith, because the Old Testament prophesied the Messiah already (It is in Isaiah 53, for example, and Daniel, and other prophetic in the Old Testament). Jesus fulfilled the law, but did not abolish it. The law is still valid. People who have become Christians, however, (real Christians, not just Christians by name, as there are millions), want to please God, because they love God, they know they owe their life to God's grace only, and so this is the motivation why they do not want to sin anymore and why they trust God enough to ask for His will.
It is not the law that forms the Christian faith, but it is the faith in God, the trust in Him, and the relationship we have with him. It is a living connection to the living God who wants to be close to us, not far away. Whomever He wants to He will attract closer and closer with His love. We Christians are redeemed – because of God's love and grace.