Sounds like it might be James Blish's [Black Easter] 1 and its sequel, The Day After Judgment, later released as a single volume, The Devil's Day, first published in Galaxy Magazine, August–September 1970, available at the Internet Archive.
Quotes from user14111's deleted answer:
The beginning:
The Fall of God put Theron Ware in a peculiarly unenviable position, though he was hardly alone. After all, he had caused it—insofar as an event so gigantic could be said to have had any cause but the First. And as a black magician he knew better than to expect any gratitude from the victor.
Nor, on the other hand, would it do him the slightest good to maintain that he had loosed the forty-eight suffragen demons upon the world only at the behest of a client. Hell was an incombustible Alexandrine library of such evasions—and besides, even had he had a perfect plea of innocence, there was no longer any such thing as justice anywhere. The Judger was dead.
The end:
Now at last, the great wings stirred slightly, and then, the three faces spoke. There was no audible voice, but as the vast lips moved, the words formed in their minds, like sparks crawling along logs in a dying fire.
[. . . .]
But eftsoons
That vacuous space where once Eternall Good
Had dwelt demanded to be filled. Though God
Be dead, His Throne remains. And so below
As 'twas above, last shall be first, and Wee,
Who by the Essenes' rule are qualified
Beyond all remaining others, must become
In all protesting agonie—the chief
Of powers for Good in all the Universe
Uncircumscribed; but let yee not forget,
Already Good compared to such as thee,
Whose evill remains will'd!
[. . . .]
I, SATAN MEKRATIG, can no longer bear
This deepest, last and bitterest of all
My fell damnations: That at last I know
I never wanted to be God at all;
And so, by winning all, All have I lost.
