This is the truth. As from a blazing fire sparks, being like unto fire, fly forth a thousandfold, thus are various beings brought forth from the Imperishable, my friend, and return thither also.

In plain words: The teacher says: this is the truth, my friend. Just as thousands of sparks fly out of a blazing fire and are made of the same fire, all living beings come out of the Imperishable, the one eternal reality, and they return to it again.