John Gill Commentary Jeremiah 51:61

"And Jeremiah said to Seraiah, When thou comest to Babylon, then see that thou read all these words," — Jeremiah 51:61 (ASV)

And Jeremiah said to Seraiah At the time he delivered the copy to him: when you come to Babylon; or are come to Babylon, to the city of Babylon, and to the captive Jews there: and shall see them; the captives; or rather the great and populous city of Babylon, its high walls, gates, and towers, whose destruction is foretold in this book, and which might seem incredible. Abarbinel interprets it of his looking into the book given him; which he thinks was not to be opened and looked into till he came to Babylon: and shall read all these words; not before the king of Babylon and his princes, and yet not privately to himself; but in some proper place, in the presence of the captive Jews, or the chief of them, convened for that purpose.