John Gill Commentary Daniel 3:18

"But if not, be it known unto thee, O king, that we will not serve thy gods, nor worship the golden image which thou hast set up." — Daniel 3:18 (ASV)

But if not
If our God does not think fit to exert his power, and deliver us, which we are well assured he is able to do; if it is not his will, we are content, we are resigned unto it:

be it known to you, O king, that we will not serve your gods ,
be it as it will, whether we are delivered or not; we are not sure of the one, but we are at a point as to the other:

nor worship the golden image which you have set up ;
come life, come death, we are ready; we had rather die than sin: they were all of one mind, and agreed in this matter; a noble instance of spiritual fortitude and courage!