# 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.