Genesis 3:15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
When we think about the coming of the Lord Jesus the reason goes right back to the beginning of time. Scripture tells us that when God put the man and the woman in the garden of Eden they only had one command. This was not to eat the fruit from the tree of the knowledge of good and evil.
In rebellion to God's command we are told that both the man and the woman ate this fruit. God had told them that in the day they ate the fruit from the tree of the knowledge of good and evil they would "surely die". As a result of eating this fruit they first died spiritually meaning that their relationship with God was now broken. They also started to die physically, meaning that one day their bodies would return to the dust from which they were created.
Because God is a God of justice He had to carry out judgment on their wrongdoing. However in judgment God also remembered to show mercy! This comes in the wonderful statement that the seed of the woman shall bruise (or crush) the head of the serpent (Genesis 3:15). God is promising that a descendant of Eve would one day undo the wrong that had been committed. The fulfilment of that promise came with the arrival of the Lord Jesus. Scripture speaks about this child of promise saying "behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us" (Matthew 1:23). The Lord Jesus is the way God has provided for us to find our way back to Him.