COnsider This . . .

If a Holy God created the human race without sin, but gave it "free choice", and then man chose to sin: that would mean that Holiness and sinfulness could not co-exist: God and man can't be together.

That is a problem

However, Love caused God to make a way to replace man's sin with His Holiness - so man can be with a Holy God. The "Why" and the "How" God did it is the story of the Bible.