When we define roles (e.g. mother, citizen, gentleman, missionary, pastor, what have you) on the basis of someone’s goodness, you lose the ability to characterize someone as a bad x.
CS Lewis commented on this in relation to the term “gentleman” - it used to mean “landowner.” Thus you could say “a gentleman and a scoundrel.” But then society changed to say “surely, the person who acts in a gentlemanly way is the true gentleman” - and thus the definition was lost.
Not all missionaries have good intentions.
Not all missionaries are well trained.
Some missionaries are lazy missionaries.
Some missionaries see their work fall apart due to terrible sins and personal failings.
Some missionaries are just bad missionaries.
None of this makes them any more or less a missionary.
It just means they have room to improve.
The same could be said for just about any role or position that we play - good and bad fathers and mothers, good and bad husbands and wives, good and bad brothers and sisters, good and bad citizens, good and bad employers, good and bad pastors, and so on.
When someone goes on a short-term mission trip, witnesses to someone, and sees that person convert - that does not necessarily make them a missionary.
We ought to be careful about calling people who “act in a missionary way,” missionaries.
People who are acting a certain way in a certain moment may resemble missionaries.
But the real test is how they act all the time, how committed they are to the craft, how accountable they are to others for improvement.
Helping someone find something in a grocery store does not make me a store employee.
I can sing and dance, but no one in their right mind would call me either a singer or a dancer.