It is important for Christians to know how they are saved. If you know and understand the Gospel you will not be drawn off into false doctrines or religions. The Gospel of Jesus Christ is the guiding light and is the light that lightens every man, John 1:9.
The Gospel is best understood in the background of the Old Testament. The Old Testament is a record of man's sinfulness. From the time that a man is born until he dies he is a sinner. Isaiah brought that out when he wrote,
"We are all as an unclean thing and all of our righteousness are like filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away" Isaiah 64:6.
If you don't see yourself as a sinner you will not understand the Gospel. The Gospel is between God the Father and his Son Jesus Christ. We had no part in it. We are the benefactors of the Gospel. Jesus comes into the world as the new Adam and the new representative of the human race. In our name and on our behalf as our representative, Jesus offers to God the Father a life of perfect obedience according to his Holy Law. It was the life of Jesus that was lived for our justification, Romans 3:26. We had nothing, absolutely nothing to do with that. When Jesus died on the cross for the sins of the world, we died with him, Romans 6:6.
When God accepted Jesus back into heaven all of humanity was accepted in him. This is why John wrote,
"And he is a propitiation for our sins: and not ours only but for the WHOLE WORLD" 1 John 2:2.
Jesus claims to be the savior of the whole world, John 12:47. As far as God is concerned his Son Jesus Christ has victoriously defeated sin, death and the devil, Colossians 2:15. Jesus now sits at the right hand of God as the "King of Kings and the Lord of Lords" Revelation 19:16. Does this mean that everyone is saved? No, this means that salvation has been provided for everyone, Hebrews 2:9, by the doing and the dying of Jesus. Jesus has reconciled us and the world unto God.
"God was in Christ, reconciling us and the world unto himself, not imputing their trespasses unto them; and has committed unto us the word of reconciliation" which is the Gospel, 2 Corinthians 5:18, 19.
Its over, its finished. The work of Christ that was done on the behalf of fallen man is finished. The ball is now in humanities court. This Gospel can be yours by doing nothing more than calling on Christ to save you, Romans 10:13.
All that accept God's offer of reconciliation are perfect and complete "In Christ", Colossians 2:10. When Jesus returns there will be a judgment of unbelievers. All that have rejected God's great free plan of salvation will perish. Their sins will be imputed back to them and they will be condemned, Romans 4:8. To escape the judgment we must be found "In Christ". God must judge sin. If you are not "In Christ" you will perish. This is why Paul wrote,
"That I might be found in him, not having my own righteousness that is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith" Philippians 3:9.