Jeremiah 2:3

Israel was holiness unto the Lord, the firstfruits of his increase Cp. Exodus 19:6; Deuteronomy 8:6; Deuteronomy 14:2; cp. Jeremiah 26:18 and Amos 3:2. There is no moral significance attached to the word holiness here. It means simply setting apart from ordinary uses, dedication to God. Israel is as the most precious part of the harvest, that part which is consecrated as God's portion. The notion was familiar through the yearly custom, prescribed Leviticus 23:10-14, that a measure of the firstfruits should be waved by the priest before the Lord, and that none of the harvest should be enjoyed till this rite had been fulfilled. Cp. Exodus 23:19; Deuteronomy 26:2 ff.

all that devour him shall be held guilty The priest and his family alone were to eat of the firstfruits. No stranger was allowed to partake. If any unhallowed person profaned the firstfruits by taking of them, he bore "the iniquity that bringeth guilt." See Leviticus 22:16 (where the Heb. root is the same as here). Thus the sense is that if unconsecrated (i.e. heathen) nations assail Israel, their fate shall be that of such as eat the firstfruits unlawfully.