en_tn_condensed/jer/02/03.md

the firstfruits of his harvest

"like the firstfruits of his harvest"

All who ate from the firstfruits were held guilty

"All who attacked Israel were held guilty like people who ate from the first fruits of his harvest"

disaster came upon them

"Disasters happened to them" or "I made terrible things happen to them"

this is Yahweh's declaration

"this is what Yahweh has declared" or "this is what I, Yahweh, have declared". See Jeremiah 1:8.