The Good News/ The Gospel
"And He (Jesus) said unto them, Go ye into all the world, and preach the gospel to every creature." (Mark 16:15) Gospel is literally translated "The Good News". What is the good news? Jesus Christ the Son of God came to Earth, was born of a virgin conceived by the Holy Spirit; lived a sinless life; was crucified on a Roman gibbet/cross; was buried and arose from the dead on the third day. "For God so loved the world that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life." (John 3:16) That's it! The Good News!
Jesus did this and took the place of his fallen creation (Mankind) to reconcile us to the Father/God. Why? Because whether you know it or not you are living in a fallen state and born into a war between good and evil. Why are we fallen? Because of our earthly father Adam (the first man found in Genesis). He willingly disobeyed God and as a result sin entered the world and God cursed Adam, Eve and the Earth. As descendants of Adam we have inherited his fallen state and are bound for Hell (a lake of fire prepared for Satan and his angels; Revelation 20) Satan is a Cherub/Angel, (Ezekiel 28:11-19) who committed the original sin "pride", and attempted to ascend above the throne of God; he rebelled against God sparking a war in Heaven involving angels of God and those that followed him; all before the creation of man. In so doing he made himself the enemy of God and therefore our (Christians) enemy. He works tirelessly to undermine God and mankind in every way. However, he has lost the war. God handed him a defeat he can never hope to regain. Where did God defeat Satan? On the cross through his son Jesus! Satan is DEFEATED and is now on a mission to wreak as much havoc as he can before the great day of judgement (Revelation 20:7-15). Man will also be judged at that time and those not found in the Lambs Book of Life will be cast into the lake of fire with the devil and his angels.
God in His mercy and grace and love toward man has a plan to redeem (buy back) mankind from this state we find ourselves in. That plan is Christ Jesus. By one mans (Adam) sin, death by sin entered the world and all are fallen (Romans 5:12), so also by one man’s gift/sacrifice/example many will be justified/reconciled to God through faith/belief (Romans 4:5) in His only begotten son, Jesus Christ. "For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, (Jesus Christ.)" (Romans 5:17). Through faith in the Lord Jesus Christ we are redeemed/reconciled/justified to God and therefore heirs to eternal life through Jesus. Faith in Jesus is accounted to us as righteousness (Romans 4:5). We have been bought back/redeemed from our fallen state and will have eternal life with God in his presence for all eternity through His son Jesus! PRAISE GOD! So how do we recieve such a great salvation? Click Here.
For a more in-depth understanding and illustration, I recommend reading a teaching "A Vivid Understanding of the Atonement".