Not to put words in Luther's mouth, who may very well have been talking about some sort of ritual mortification of the flesh, such as self-flagellation or some other "discipline". However, I have also heard more contemporary scholars discuss that while salvation may come from faith, not works, a purely inner repentance is meaningless unless it is demonstrated that a person is making efforts to amend the sinful behaviour.
Well, as usual with Luther, the German is very in-your face, '
...nicht nur auf eine innere Buße, ja eine solche wäre gar keine, wenn sie nicht nach außen mancherlei Werke zur Abtötung des Fleisches bewirkte.'
Worldviews have changed enough in 500 years so it does need to be parsed ... when he demands, literally,
'various outwardly directed works that serve to deaden the flesh' this is obviously of course not about the literal destruction of tissue in your body, nor do I think it's to be limited to ritualized pretenses of 'mortification'.
Absolutely uncompromisingly though he does demand 'Werke' i.e.
works to accompany inner repentance, and they must be directed
outwards.
Also it's important to remember that Luther wasn't aiming at introducing a more 'tolerant' Christianity, but to re-radicalize it, so his intent is not always something that we'd find 'enlightened' or progressive today.
'
The flesh' is the sinful carnal nature as such -- at the least the sum entire of ungodly desires, drives, and behaviors, and 'deadening the flesh' is about attempting to make yourself 'dead to sin'.
It's a concept that's repugnant to me because I don't subscribe to that way of thinking on which parts of human nature are good or bad, I don't agree with assigning evil to 'the flesh' but that's the background.
An interpretation that's palatable for modern tastes would be that any spiritual insight to change some destructive or toxic behavior you've been engaging in, has to be accompanied with a visible according lifestyle change, that should come at some inconvenience, effort or cost.
Modern example! -- A person comes to care deeply about The Planet, and climate change and such. Finding inner repentance in recognizing the wrongness of how resource consumption and resulting pollution will mess up the world for unborn generations. But, she continues to fly by 747 to conferences where everything is said about the topic that's been said for 25 years, gets new Iphone, Ipad, Imac every year and drinks imported plastic-bottled water -- so, her inner repentance is not accompanied by outward 'mortification' i.e. accepting a quantifiable and publicly visible 'sacrifice' with her repentance. Basically it's ... hypocrisy.
Repentance is a concept from religious teaching, yes, but it is not necessary to be religious to repent of something.
I absolutely agree with that.
Obviously today our valuation what kinds of thoughts, desires and behaviors are 'sinful' and need repenting from have changed quite a bit but even among the very secular, the idea of sin which is inherently bad as such even before it becomes a deed is alive and well. It's just expressed with different language.
OK ! Everybody is commiting errors , but I dont see why we should have to suffering for them !
We can try to repair them but why to have a pain for them ? Are the victims happier in seeing you suffering ?
If yes, that's a pity because their detriment will not be changed after your punishment ...
I don't think that the intent is to make
other people happier through seeing a 'sinner' suffer, rather that
choosing to suffer is an expression of that step of one's development, and that it can make oneself more secure, and happier, resting in that change of course because it's not just a thought your mind went through but also an effort or pain your body went through, and so it's become part of your being, your instinctual self as well as your conscious self.
That, in some way, makes sense to me.
in fact, getting whipped to orgasm is a perfectly fine purification the way they see it 
