The formless god because of his immense love towards us manifests himself on earth in different forms. in the form of the guru he directs us such that when we follow him with faith we merge with the almighty. hence there will be no further birth. we do not have messengers because our sanathana dharma believes that god does not like middlemen. as the geethacharya he told us the importance of swadharma -aacharna( sri krishnam vande jagadgurum). as janaki ramana he told us how must lead we must live holding steadfast to dharma( ramoh vigrahavvan dharmahah). as sri sri sri chandrasekharendra saraswathi he again and again reiterated the importance of swadharma acharana through adwaita. all these walked on earth and sanctified it. all we have to do is follow them with sraddha.