Martin Luther's discovery of the doctrine of justification by faith alone.
I’m sure many of you are celebrating Halloween today (or yesterday, depending on the discretion of your local municipality), but for many Christians, Oct. 31 marks the 493rd anniversary of Martin Luther ushering in the Protestant Reformation by nailing his 95 Theses to the door of the castle church in Wittenberg, Germany. And this year the anniversary happens to fall on the Lord’s Day.