After King Darius declares that the God of Daniel should be worshiped in all of the land of Persia, does that mean that he is saved?
In Daniel 5, the narrative fast forwards about 23 years into the future. Five kings pass to arrive at Belshazzar. He is the second that Daniel serves. Daniel interprets the writing on the wall for him. But as prophesied, he is the last King of Babylon.
Babylon falls to King Darius of the Persians. This is the third King that Daniel serves while his people are in exile from Israel. King Darius is very fond of Daniel and he rises in power. This causes the episode of the lions’ den.
King Darius is so surprised and astounded by the God of Israel protecting Daniel overnight in the midst of hungry lions that he declares that Daniel’s God should be worshiped throughout his lands (Daniel 6:25-28).
All we are told is that Daniel prospers through his reign and the reign of King Cyrus. Then in Daniel 7, it returns to King Belshazzar of the Babylonians. It doesn’t return to the reign of King Darius until Daniel 9. He is not mentioned again in the book of Daniel.
Many of the ancient Kings who are not Israelite kings tended to waiver between different gods. Being polytheistic, they can worship multiple gods at one time. But Israel’s God is exclusive. You cannot worship him and serve other gods as well.
It is most likely that King Darius may have had a fleeting moment at the end of Daniel 6 when he saw what Daniel’s God could do. He may have enjoyed for a short time veneration of Daniel’s God, but it is most likely that he would either include the God of Israel in his list of gods instead of truly worshiping God exclusively. It’s not likely he became a follower of the God of Israel or a convert of the Jews.