[Boromir88]

Tolkien wrote in his letters that Gandalf was the only one to stay true to the "mission" of uniting the free peoples. I have some assumptions on Radagast, however, since he was sent by Yavanna his mission could have been much different. Being that it was Yavanna that sent him it makes sense Radagast would fall in love with nature, and take care of its animals, and his mission could have actually been to do that, and to aid Gandalf if he needed it. As for the "mission" of uniting the free peoples and to stand against Sauron, yes, only Gandalf stayed true to that cause. Not enough info was said on Alatar and Pallando, their involvement could of very well had something to do with lets say bringing down Sauron, but that is only speculation, it is most likely they just went into the East and did their own business.