Celestial Envoys are the messengers of the gods. The gods cannot live on unless people believe in them. We Poncles are the only ones who can communicate with gods. And we train as artists to teach the world their divine power.
Poncles are full of artistic talent and a strong willed loyalty to the Gods of Nippon. When the people of Nippon forget or lose faith in the gods, Ponc'tan sends out a Celestial Envoy (「天道太子」?; Tendō Taishi) to travel around the world painting, drawing, and reminding people that the gods still live and protect the people. Poncles also have the ability to speak with many if not all of the animals of nature.
The gods themselves rely on the Poncles to tell the people because the love and faith from the people all over Nippon is what gives the Gods their true power. Issun is the current Celestial Envoy (the 7th Celestial Envoy), and his grandfather, Ishaku, was the Celestial Envoy 100 years ago (the 6th Celestial Envoy).
Issun was acknowledged by the people in Ponc'tan to be an artistic talent. Because Ishaku had really expected Issun to be the next generation Celestial Envoy, his training regime with Issun is much more strict than that of the other artist in-training. This caused Issun to develop a hatred painting, and on one day, he stole a painting of Sakuya made by Ishaku and left Ponc'tan.
Until the end of Amaterasu's journey, Issun rejects his fate to be a Celestial Envoy regardless of the fact that he had learnt all of the Celestial Brush techniques. His ignorance lead to Amaterasu's lonely venture into the Ark of Yamato. Although in the very end, Issun accepted his destiny as a Celestial Envoy and spreads his paintings of Amaterasu all across Nippon so that the people will believe and pray for the gods. This act of Issun have revitalized Amaterasu during the final fight with Yami.
Spoilers end here.
It is unknown who the 1st, 2nd, 3rd, and 4th Envoys were, but it is known that the 5th Celestial Envoy was Ishaku's great-grand father.
Ishaku was the person who wrote all the Travel Guides, scattering them around Nippon. One particular scroll discusses about the destiny of Celestial Envoy themselves.
The Celestial Envoys' job is very similar to those of the prophets in Islam. Slightly different, their role is to draw pictures of gods to evoke believe and faith rather than spreading true teachings.
↑"A Celestial Envoy's job is to paint pictures of gods. They help rally the people's faith, ya know? When people start to lose faith, the gods grow weak. I'm the seventh official Celestial Envoy! I travel up and down the land, spreading the word. It's a tough gig, but I'm gonna keep doing it -- until someone chooses the eighth Celestial Envoy!"—Issun, scroll Celestial Envoy (Ōkamiden)