"Gospel" is the English translation of euaggelion (ευαγγέλιον), a Greek word that means "good news," so the true Gospel means the true good news. The myriad half-truths that parade as the "gospel" makes knowing the true Gospel imperative. And to know the good news, we need to start with the bad news. Here is the true Gospel in plain language.
1. God is perfect and so is His law, which we cannot keep since we are imperfect.
2. Any gap between God's perfect law and our imperfect obedience is called "sin," which everyone has committed: "For all have sinned and fall short of the glory of God" (Romans 3:23).
3. The penalty God mandated for sin is death: "For the wages of sin is death, ..." (Romans 6:23).
We may think we are good people according to our standard of goodness, but according to God's much higher standard of goodness (and He judges us by His standard, not ours), all of us are convicted death row inmates. This is the bad news.
What is the good news?
Because God loves us, He didn't want to wipe us out. But because God is good, He couldn't simply brush aside our sin either.
Why not?
Imagine something horrific. Imagine finding a man raping and murdering your mother. Instead of exacting vengeance on the murderer, you drag him off to a judge and demand justice. If the murderer is convicted but the judge declares himself a merciful person and simply frees the murderer, would you consider him a good judge who upheld justice?
Of course not. A good judge must punish crime according to the law. Likewise, God, who is perfectly good, must punish sin according to His perfect law. Otherwise, He wouldn't be perfectly good - i.e., "Holy."
So what did the Holy God do about our sin?
He hurled the punishment required by His law - the death penalty - at us, and then stepped in front of us to take the full brunt of that punishment.
If that is tricky to understand, imagine you committed a crime that carries the mandatory death penalty and have been convicted. You rise at your sentencing and the judge hands down the death penalty. As you start to tremble in fear, the judge rises from his seat, walks down to you, and tells you:
"Because I am a good judge, I sentenced you as the law requires. But I ... am also your father and I love you. And because I love you, I will take upon myself the death penalty you deserve, and grant you life instead of death."
That is why Jesus engineered His own death on the cross: "I am the good shepherd; and I know My sheep, and am known by My own. As the Father knows Me, even so I know the Father; and I lay down My life for the sheep. And other sheep I have which are not of this fold; them also I must bring, and they will hear My voice; and there will be one flock and one shepherd. Therefore My Father loves Me, because I lay down My life that I may take it again. No one takes it from Me, but I lay it down of Myself. I have power to lay it down, and I have power to take it again..." (John 10:14-18).
The details of how meticulously Jesus engineered His own death on the cross are here.
What happens to people who believe that Jesus did this for them?
They will have everlasting life in heaven - "For God so loved the world that He gave His only Son, that whoever believes in Him should not perish but have everlasting life" (John 3:16) - and this is why Romans 6:23 above concludes, "For the wages of sin is death, but the gift of God is everlasting life in Christ Jesus our Lord."
What happens to those who don't believe this?
God said they will pay the penalty for their sins on their own in the fire of hell: "The Son of Man will send out His angels, and they will gather out of His kingdom all things that offend, and those who practice lawlessness, and will cast them into the furnace of fire. There will be wailing and gnashing of teeth" (Matthew 13:41-42).
What about going to heaven by doing enough good works?
If an Olympic athlete wins a gold medal and then rapes someone, will his gold medal performance exonerate him of rape? The issue is sin and justice, not works.
Then why do Christians do good works?
Not to gain salvation but to thank, honor and obey Jesus, who saved us and commanded us to do good works. When you walk outside on a sunny day, your shadow follows you. Your shadow does not produce your body; your shadow proves that your body is real. Likewise, good works do not produce salvation; good works prove that your faith in Jesus as your savior is real. People who do good works thinking that good works save them reject Jesus' salvation and sacrifice. We are saved not by what we do but by what Jesus did for us on the cross.
Do Christians continue to sin?
The Holy Spirit takes up residence inside true Christians - "your body is the temple of the Holy Spirit who is in you" (1 Corinthians 6:19) - and transforms them from the inside so that they come to hate and repent from the sins that they used to love: "Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new" (2 Corinthians 5:17). Watch the video below.