Writing Discussion

I studied the passage 622-662 of Paradise Lost. My question was (focusing on line 649: "...hath overcome but half his foe"):
If the Devil was only one half of God's foe, what is the other? Is it man (the "new worlds" in line 650)? If it is some other beings, like man, what are the implications of them being the enemies of God?

The responses to my question were varied, though they all agreed that half of God's foe was the Devil and the other half represents some devilish quality, in either man, God himself, or the actions of the Devil. The first response suggests that the other half was the actions the Devil takes through man, the second says that the devilish qualities that should exist in God (because no one is completely good or completely evil) are the second half of his own foe, and the third suggests that because God used force to defeat/suppress the Devil, he has not overcome all of his foe. I like that my question provoked so many varied and interesting responses and I think all of them have validity and are capable of being supported. When I wrote the question, I assumed that the other half of God's foe was man, but with these responses, I can think more about what Milton could have meant.