If salvation isn't a legal transaction, what is it?

It is being made alive again, not just being declared not guilty. The picture the Bible opens with is Genesis 2:7: a body of dust lies lifeless until God breathes into it, and only then does it become a living being.

That is the shape of salvation. Sin is first a broken relationship with God, not first a legal charge, and being saved is that relationship being restored, the breath put back in. It is not only a future ticket to heaven; it is a present, real change in a person now. So salvation is less like winning a court case and more like something dead being brought back to life.