The Sons of Kaleva is the holy order for the Suomenusko faith, and can be used to fight enemies of other faiths. They get their names from the giant Kaleva, father of the Suomenusko gods.
As with the other (non-Norse) pagans they are formed by reforming Suomenusko, and having the Sons of Abraham DLC.
Kaleva is similar to Ymir in the Norse religion. He was revered as the father of the gods, and original creator of the Earth. The Suomenusko epic, the Kalevala was named after him.