To know who Jesus is and to understand what and why He did what He did, we have to go back to the beginning. God created all there is, including the earth and everything on it. God made mankind to be perfect in every way and to have fellowship with Him. God didn’t want little robots running around, so He gave people a free will in order to decide for themselves if they wanted to be God’s friend or not.
Adam and Eve were the first two people and the only ones to be directly created by God; before they had any children, they chose to disobey God and to use their free will to commit sin by doing the one thing God told them not to do—eat of the fruit of the tree of the knowledge of good and evil. When they disobeyed God, a sin nature was developed in them; a sin nature that was then passed down from parent to child in a continuous line until this day. Sin means “missing the mark”; it’s both the actual act of disobeying God (such as stealing, lying, murder, etc) and the result of that disobedience. See Romans 3:23.
Even though humans sinned and caused a separation between themselves and God, God still wanted the companionship He set out to get from the beginning. But being a Holy God, by His very nature, He is unable to even being the presence of sin. Think of the nastiest, most disgusting place you can think of, multiply that times a million and that’s not even close to what God thinks of sin.
But, because He loves us so much, He came up with a way for us to come to Him and reclaim that fellowship. He needed someone to pay the penalty for our sin. We couldn’t do it because we have committed sin and are an unclean payment. God says that the penalty of sin is death, so that only payment He will accept is blood, but not just any blood, He needs clean, holy blood, to wash away the sin of the world. See Romans 6:23.
This is where Jesus comes in. God is a Triune Being; He is one God in three Persons: the Father, the Son, and the Holy Spirit. The Son, Jesus, left His heavenly home, took on human flesh, and was born to Mary and Joseph. He grew up and became a normal man, with one exception--He never sinned! Although He was tempted to sin just as we, not once did He submit to any temptation, thereby becoming the perfect sacrifice for our sin. God accepted Jesus as a substitute for all of us. See 2 Corinthians 5:15 and John 14:6.
God will now let us into His heavenly home, to live with Him for all eternity, if simply believe in Jesus as the only begotten Son of God, that He lived a totally sinless life, that He died and shed His holy blood in our place, and that 3 days later He arose from the dead and ascended back to heaven to become our mediator before God the Father. If we ask God to forgive our sins, they will be covered by Jesus’ blood and no longer seen by God! We are redeemed and seen as holy if only we truly believe. See Romans 10:9-11.