Gylfaginning - a part of the Younger Edda, is one of the most important scriptural sources in defining the Norse worldview.
Gylfe, a mythical king of Sweden, wondered how the gods were so knowledgeable, that everything went according to their will. To explore how things were with this, he undertook a journey to Asgard. He assumed the guise of an old man and called herself Gangleri and believed he in this way able to remain unknown. They therefore made the following illusion. When Gylfe arrived in Asgard, he thought he saw a high palace, and when he entered the hall, he saw three seats of honor, one above the other, and a man sitting in each. The one who sat in the bottom of honor, called the High, the closest to him uniformly high, and the colonel of the Third. They asked Gangleri on his errand, and he said he would like to know if he was a remarkably clever man. Thereafter a conversation takes place; Gangleri asks and the others respond. The topics is about the Norse mythology, from the world's creation to its destruction. When the call was over, Gangleri heard a huge uproar on all sides. He looked around and found himself all alone on a flat and vast plain, but he was now no palace and no castle. He made his way home and reported what he had heard and seen.
Lets hypothesise a bit for a moment and ask if this story contains a grain of truth. Lets say that this story is about a man traveling east of Black Sea and discovers different religions on his way. He tries to make sense of it and tries to explain it to his fellow norse-men.
Do we have anything supporting this idea? Yes, Jacob Grimm, is mentioning the one of the gods Austri might actually be the goddess Ēostre.
In that case, how do we know for sure that these gods mentioned in Gylfaginning where actively worshiped or even commonly known by the norse vikings?