Well, it all starts with God. God is the only God, the holy and just creator of the universe who created us for his glory (Rev 4:11); that is, we were created to live lives that show off the glory, perfections, and beauties of who God truly is (1 Cor 10:31).
But we have all failed to glorify God in this way. This is what the Bible calls 'sin' (Rom 3:23).
What is sin? Sin is a failure to be perfectly righteous like God... in how we act, in our attitude, in our nature.
We are sinners because of what we do: we break God's commands.
We are sinners because of what we love and desire: we desire creation, not the Creator.
We are sinners because of what we are: we are humans born from other sinful humans.
Many people say, "I'm a good person, I do good things, God will forgive me." But the Bible says that "all our righteous deeds are polluted garments" (Is 64:6). That is what our best works are worth when we stand before God to be judged. To hand your kindness, faithful church attendance, memorized Bible verses, sincerity repeating the sinners prayer, tears, mission trips, etc, to God in hopes of receiving forgiveness from him is actually an offensive to him.
So the morally clean church member and rebellious God-hater are both guilty before God as sinners, and fully deserve eternal death and torment under God's righteous wrath in hell (Rom 1:18).
That may sound harsh because it is. "The penalty for sin is death" (Rom 6:23). Even one sin against an eternal God is fully deserving of an eternal punishment.
You might be thinking, "Where's the good news in that?" But that is not the good news. Here is the good news: Jesus.
The good news is this:
We are guilty, but Jesus is innocent.
We are sinners, but Jesus is sinless.
We failed to glory God, but Jesus glorified God - perfectly!
Because Jesus was the commandment keeper, he can save commandment breakers. The Bible says, "while we were still sinners, Christ died for us" and that therefore "we shall be saved by him from the wrath of God" (Rom 5:8-9).
Jesus was literally "God damned" in our place. Here's how this works... "For our sake he (God) made him (Jesus) to be sin who knew no sin, so that in him we might become the righteousness of God" (2 Cor 5:21).
Some call this "The Great Exchange." Jesus gets our sin, we get Jesus' righteousness. The mortal blow of God's wrath that should have crushed us instead crushed his Son, and "it pleased the Lord to crush him, when his soul makes an offering for sin" (Is 53:10).
And then Jesus proved that he made the offering for sin and accomplished our salvation by rising from the dead (1 Cor 15:4). The resurrection is the Father's stamp of approval upon the death of his Son. This is good news!
The Bible says, in light of what God has accomplished in Christ, we should respond with repentance and faith (Mark 1:15). Our response should be "repentance toward God, and faith in Jesus Christ (Acts 20:21).
Repentance is turning away from sin and turning to Jesus. "Repent, then, and turn to God, so that your sins may be wiped out" (Acts 3:19/26:20).
Faith means we trust that the death of Jesus fully satisfied God's wrath on our sins. "Believe in the Lord Jesus and you will be saved" (Acts 16:31). When we trust and treasure all of who God is for us in Christ, God is glorified in us and we are satisfied in him - forever.
Jesus said, "He who believes in me, shall never thirst" (Jn 6:35).
Some people think forgiveness is the greatest thing about salvation, but forgiveness is only a means to an end.
Forgiveness makes it possible for bad people to be brought back to a good God. "For Christ suffered once for sin, the righteous for the unrighteous that he might bring us to God" (1 Peter 3:18).
The good news of the gospel is that we get to enjoy God forever, the very thing for which we were originally created.