Imagine the reaction to this scenario. One day your nice little son or daughter comes home from school and says, "Today we learned to pray the Hail Mary!" You see, your kid's teacher that year is a nice Roman Catholic, and as such, out of her very sincere and devout religious beliefs, for the good of the children, she teaches them the Hail Mary.
Would you applaud this? Approve of it? Say that it is a good thing?
No - you'd probably be really upset, and why? Because the teacher exceeded the limits of her vocation. In her vocation as teacher (either at a public school or even if she's a nice Roman Catholic gal with a teaching gig at a Lutheran school), teaching the Hail Mary goes beyond what she has been given to do. It doesn't matter if that is her sincere belief - it doesn't matter if she is standing up for her faith. She went beyond her vocation.
Vocations set limits. They define what actions you can take and who you take them with. I am a father - that gives me a whole set of responsibilities and duties -- but to my children. I am a pastor - I am called to preach God's Word - a good thing. But if I barge into a congregation not my own and demand to preach, I have exceeded my vocation - I have no call to preach there (no "diploma of vocation" there, if you will).
We forget that our vocations have limits. If you have an office or duty - that's a good thing... but you are called to carry out the duties of that office. And that's it. You don't assume unto yourself duties not given to you. That's the heart of enthusiasm, that's the Zwickau prophets and their error. That is running when not sent by the Lord.
This idea of vocation setting limits is all over the place in the Catechism - it's the heart of the table of duties... your station in life determines what you are *or aren't* to do. Another application of it that I think of comes from the Large Catechism on the 8th Commandment: "Therefore, to avoid this vice we should note that 265]
no one is allowed publicly to judge and reprove his neighbor, although
he may see him sin, unless he have a command to judge and to reprove. 266]
For there is a great difference between these two things, judging sin
and knowing sin. You may indeed know it, but you are not to judge it. I
can indeed see and hear that my neighbor sins, but I have no command to
report it to others. Now, if I rush in, judging and passing sentence, I
fall into a sin which is greater than his. But if you know it, do
nothing else than turn your ears into a grave and cover it, until you
are appointed to be judge and to punish by virtue of your office."
Did you see how often "office" came up? Even when it comes to spotting sin - unless your job is to punish and reprove sin - you don't punish and reprove it. This is why we have police - not vigilante mobs (sorry folks, but Batman is not acting in accord with good Lutheran theology... at least until the city puts him on de facto retainer with the batsignal, I suppose). This is why we have judges.
Thus, when looking at what you do, the question is not merely "what is right" but rather "what is my vocation?" If you do something that is abstractly "good" or that you can argue for, but it's not your vocation... then it is fundamentally bad and should not be encouraged, even if good comes from it.
The ends do not justify the violation of ones office.