Jamieson, Fausset and Brown on Daniel 6:16

16. Thy God ... will deliver thee--The heathen believed in the interposition of the gods at times in favor of their worshippers. Darius recognized Daniel's God as a god, but not the only true God. He had heard of the deliverance of the three youths in Da 3:26, 27 and hence augurs Daniel's deliverance. I am not my own master, and cannot deliver thee, however much I wish it. "Thy God will." Kings are the slaves of their flatterers. Men admire piety to God in others, however disregarding Him themselves.