In contrast to Rehum and Shimshai's letter to Artaxerxes (4:11-16), Tattenai's letter to Darius was fair and objective. He did not want to stop the Jews' project. He only wanted to know if Cyrus had really given permission for the Jews to rebuild the temple and if Darius wanted that edict to stand.
The record of this letter in the text shows that high ranking government officials had observed God's care of His chosen people. This would have encouraged the original readers of Ezra with the assurance that what they had done was honest and that God was moving governors and kings to accomplish His will.
". . . against the background of rampant polytheism or even the dualism of newly emerging Zoroastrianism it was important to affirm that Yahweh is Lord of all in heaven and on earth. To their enemies the Jews affirmed this when they announced that they were building the second Temple as the servants of the God of heaven and earth' (Ezra 5:11)."87
"The God of heavenis probably an attempt by the Jews to create sympathy for their cause in the Persian court, because Ahuramazda, the Persian god, was also regarded as god of the heaven,' and was known as the creator of heaven and earth."88