Sweetheart, Kant is one of the giant’s of philosophy - of course I’m familiar with his categorical imperative. This thread is about the protestant work ethic, not Kant’s moralisms. I know you like to keep it terse, but sometimes you need to elaborate more, to avoid ambiguity.
It evaluates actions by considering the outcomes of engaging in a behavior if everyone else would behave like that as well.
Example: If everyone lied and cheated, nobody would trust anybody and conducting any kind of business would be impossible. Buying/selling would be impossible, trading would be impossible, working for a salary would be impossible, providing a service for reward/payment would be impossible.
There would be no work to be done outside of working for your own food off the ground in your own land and shooting any trespassers.
If you have no land to work on, I guess that’s tough luck, better luck next lifetime.
So there, just like mommy told you. What would you think if they did that to you? What if everyone did it to everyone?
As far as a motivation to engage in work ethics, the obvious is that anyone conducting business with you must be able to trust you. Cultivating an image of trustworthiness is part of developing a career.
In reality it is not trustworthiness itself that matters, but the ability to impress trustworthiness on people. If you work hard and nobody ever sees it, or are unable to tell by the results of the work, what good is that to you?
In sum: Is it protestant? Judeo-christian? Kantian? Aristotelian? What-fucking-ever. It is what it needs to be in order to make business feasible.
Yes, it’s common sense that in order to acquire/maintain work, you need to have a reputation of trustworthiness, etc. But the OP hones in on something more specific: the Protestant work ethic, i.e., the deification of work ( work itself as a virtue ) originating from Calvinism and subsequently, leading to the creation of secular capitalism.
Your initial post was simply incorrect and your second - the Kantian one - was off the mark.
Why would you bring that up, knowing that it’s not germane to the OP? Even so, you are still incorrect in stating that diligence has nothing to do with work-ethic. That’s straightforwardly false.