The gospel, which literally means 'good news' can be simply stated as Believe in the Lord Jesus, and you will be saved (Acts 16:31). It is so easy that children can believe it, yet so complicated that learned men are not able to understand it. God is holy, we are sinful and our sin is keeping us from God. God has provided a way through Jesus whereby we can be reconciled to Him - this is the good news. If we believe in Jesus, we will have our sins forgiven and when we die we will go to heaven. If not, we will go to hell for eternity.
God is holy and we are sinful
Holy, holy, holy is the Lord God Almighty (Revelation 4:8). God created the first man, Adam, in His image and he was therefore good. When Adam sinned, his good nature was corrupted and he had to die. This corrupt nature is inherited by his descendants and so all men sin and all men die. There is no-one righteous, not even one (Romans 3:10). Because God is perfectly holy, every sin, no matter how small, has to be judged fully. If we die in our sins, we will be permanently separated from God and subject to His anger for eternity.
We need an atonement for our sins
If we are to enter into God's presence, we need to be washed thoroughly clean from every one of our sins. God is loving and merciful and so He has provided a way so we can do this. It is futile to ask God for forgiveness without an atonement for our sins. God has to express His anger at sin and an atonement is something which takes the punishment due us on our behalf.
Jesus Christ is the atonement
For there is one God and one mediator between God and men, the man Christ Jesus (1 Timothy 2:5). Men have tried to make peace with God using their own methods. Some try to obey the law written in the Bible as best as they can and hope that God will overlook the times they fail. But no-one is justified before God by the law (Galatians 3:11). Others give offerings or sacrifice animals for their sin. But these are of less value than man and so cannot be substituted for us.
Only Jesus Christ can pay the penalty for our sins. He was born of the Holy Spirit and therefore did not take on Adam's sinful nature.
He was tempted and did not sin. If He died a sinner then He would share the same condemnation as us. He did not have to die but offered His life willingly on our behalf. He is the Son of God and therefore His life is of greater value than all men who have ever lived, added together. God confirmed Jesus was an acceptable atonement by raising Him from the dead.
The atonement does not automatically apply
There is a judgment day coming when God will judge all men. If the atonement applied automatically, then it would completely undermine this day, because everyone would become guiltless. God requires us to respond. 'Today, if you hear his voice, do not harden your hearts' (Hebrews 4:7). In Mark 1:15 Jesus proclaims the gospel. 'The time has come,' he said. 'The kingdom of God is near. Repent and believe the good news!'