This is regarded as the start of the Reformation- his protests against the Pope, and the Catholic church, brought about the 'Protestant' faith. Luther had come to understand that 'eternal life' wasn't something you could earn by good works, or buy from a priest
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He had found that eternal life was a free gift of God's grace, available to all, and that this abundant life began straightaway, not after death. He wasn't popular with the Pope and the other Catholics.
He was called upon to recant his heresy. But he wouldn't [Here I stand...] so was excommunicated- cut off - from the Church of Rome. Once he was no longer a celibate priest, he married Katherine von Bora, herself an ex-nun.
People have got rather carried away with the celebrations [rightly so, imho] although some of the memorabilia is ...unusual. My Lutheran friend has been to the celebrations in Germany this summer, and returned with some Here I Stand socks[hopefully not too holy?].
You can also get life size figures of Martin and Katherine to decorate your home[tacky], a playmobil Martin for the children [cute], and much more...
In 1967, when it was the 450th anniversary [much more muted - that was pre Internet, so merchandising and publicising was harder] I was in Grammar School. Our History Master [Mr Watson, aka Pugwash] set his annual competition to write a biography of a historical character.
There were two winners that year, my Methodist mate Deborah, was one, with her bio of John Wesley - and I shared the prize, with my bio of Martin Luther.
Here are a few good Luther Quotes - you can find plenty of others online [but just be careful - some uininformed people are confusing Martin Luther with the Baptist Preacher born 400 years later, and named after him - Martin Luther King!!]
I rather like this one
And this one
But this one really challenges me