Albert Barnes Commentary Jeremiah 11:5

"that I may establish the oath which I sware unto your fathers, to give them a land flowing with milk and honey, as at this day. Then answered I, and said, Amen, O Jehovah." — Jeremiah 11:5 (ASV)

As it is this day – God had kept the terms of the covenant. Whether the promised land would permanently remain the property of the Jews would depend upon their observance of their part of the covenant.

So be it, O Lord – Or, “Amen, Yahweh.” The prophet was literally obeying the command given in (Deuteronomy 27:14–26), and the same word should be kept in both places.