In the Norse myth of Ragnarok the prophecy is that, at the end, the gods will fight the giants; and will lose. So why do the gods fight?
The gods fight because that is how they win.
The gods are Men - the giants are demons.
Ragnarok is the final battle of the spiritual war on earth.
And Men will lose the last battle - on earth.
(How could it be otherwise? The giants are just too powerful.)
But choosing to fight against the giants, the demons; we win in Heaven, and eternally.
So - like the Norse gods; we prepare for Ragnarok.