Charles Spurgeon Commentary Daniel 6:15

"Then these men assembled together unto the king, and said unto the king, Know, O king, that it is a law of the Medes and Persians, that no interdict nor statute which the king establisheth may be changed." — Daniel 6:15 (ASV)

This is the reason for his deliverance, not his innocence, but his faith; we are told by Paul that it was faith that shut the mouths of lions.