Everything about "pox leber" as a minced oath, and an earlier source.

[Rafal]

I first said that people resorted to minced oaths because the Church didn't like them swearing by God. But this sermon voices the opposite sentiment.

The second Commandment of God says "Thou shalt not take the name of the LORD thy God in vain".
So you should use the name of God in serious matters but shouldn't overuse it and use it in triffle matters.

And people were probably doing exactly the opposite - talking about God all the time but avoiding it when it involved some serious commitment.