Euphemisms. "Teaching" is nice way of saying, proselytizing. "Teachers" are proselytizers. "Oneness of humanity" is when most, if not everyone, converts to the Baha'i Faith. "Service" or "service to humanity" is almost always some sort of religious work and most religious work is "teaching work". "Pioneers" are long-term missionaries. "Unity" has the same meaning as "oneness of humanity", which I briefly mentioned. "Oneness of religion" doesn't mean that all religions are one; it has the same meaning as in Islam, in that, most of the predominant religions were revealed from God, but are now outdated and distorted. When I left the Baha'i Faith, it took me a long time to rewire my brain as to the real meanings of these terms. It was frustrating.
http://bahaism.blogspot.com/