So what's the deal?

In the Bible, a guy called Paul explains it like this:

'What Moses and the prophets witnessed to (in the Old Testament) all those years has happened. The God-setting-things-right that we read about has become Jesus-setting-things-right for us. And not only for us (Paul is writing to Jewish Christians here), but for everyone who believes in Him. Since we've compiled this long and sorry record as sinners - both us and them - and proved that we are utterly incapable of living the glorious lives God wills for us, God did it for us. Out of sheer generosity he put us in right standing with Himself. A pure gift. He got us out of the mess we're in and restored us to where he always wanted us to be. And he did it by means of Jesus Christ.
God sacrificed Jesus on the altar of the world to clear that world of sin. Having faith in Him sets us in the clear. God decided on this course of action in full view of the public - to set the world in the clear with Himself through the sacrifice of Jesus, finally taking care of the sins He had so patiently endured. This is not only clear, but it's now - this is current history! God sets things right. He also makes it possible for us to live in His rightness.'