You often hear testing proponents (such as myself) lament their non-testing peers with an almost religious fervor. That is often counter-productive but more importantly; the case can be made way more tangible than just promoting The Right Way™.
The reality is that your code will be tested whether you want it or not. Either by you, your peers, your supervisors, your clients or in the very end; your customers.
I can live with bugs being found by my peers but the further away from me the bugs are found the worse I feel about it. Of course I would feel much better if I never created any in the first place but writing code inevitably means writing bugs. Thinking else is naive.
With today’s excellent testing frameworks, whatever your development environment, there really is no excuse. Take the time, learn how to test and get to it.
Do not let your customers test the code for you.