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Line. 351-353: So was his will.....circumference, confirmed.
Beelzebub was the last of those who debated on the issue what the fallen angels should do to wreak their vengeance on God. His plan was not an invasion of Heaven or a policy of inanition like Belial’s or the founding of an Empire like Mammon's but an excursion into the new worid created by God, and the seduction of the new race called Man. Outlining his plan, he reminded them that while they were in Heaven they had heard of an ancient prophecy that a new world would be created, and God Himself had made an announcement about it in His assembly. He refers in these lines to this declaration, and says that God affirmed His intention by a vehement oath, so that when He swore it, the whole of Heaven shook in all its circumstance.
The point of interest about his reference to God's declaration is that it appears to be inconsistent with what Milton has described later in the poem through the lips of Satan that God seems to have created the Universe to fill the void created by the angels falling from Heaven. If the creation had thus been contemplated then it could not have been known either to Satanor Beelzebub. But Milton sets all doubts at rest on this point by mentioning a few lines earlier than this context that the creation of the Universe had been foretold by an ancient prophecy, which implies that it must have been made long before the rebels fell from Heaven. Later, when God confirmed it by His oath, it must have been with the fore-knowledge absolute "that these angels would rebel and that the void left by them in Heaven should be filled by Him otherwise. Dr. Johnson makes considerable ado about this supposed inconsistency.
It appears rather strange that God should swear an oath, but we have the authority of the Bible to show that when God desired to assure His hearers, and make confirmation strong He did swear. Besides Milton may have here in mind a line in Homar which represents Zeus as shaking Olympus by the expression of his will.
Lastly, the sentiment that Heaven shook round its circumstances to the vehemence of God's oath is illustrative of Milton's love of magnificence of any kind, here, a great volume of sound.