Deuteronomy 29:18

"That there may not be among you," etc.: this sentence may be easily explained by introducing a thought which may be easily supplied, such as "consider this," or "do not forget what ye have seen, that no one, either man or woman, family or tribe, may turn away from Jehovah our God." - "That there may not be a root among you which bears poison and wormwood as fruit." A striking image of the destructive fruit borne by idolatry (cf. Heb 12:15). Rosh stands for a plant of a very bitter taste, as we may see from the frequency with which it is combined with לענה, wormwood: it is not, strictly speaking, a poisonous plant, although the word is used in Job 20:16 to denote the poison of serpents, because, in the estimation of a Hebrew, bitterness and poison were kindred terms. There is no other passage in which it can be shown to have the meaning "poison." The sense of the figure is given in plain terms in Deu 29:19, "that no one when he hears the words of this oath may bless himself in his heart, saying, I will prosper with me, for I walk in the firmness of my heart." To bless himself in his heart is to congratulate himself. שׁרירוּת, firmness, a vox media; in Syriac, firmness, in a good sense, equivalent to truth; in Hebrew, generally in a bad sense, denoting hardness of heart; and this is the sense in which Moses uses it here. - "To sweep away that which is saturated with the thirsty:" a proverbial expression, of which very different interpretations have been given (see Rosenmller ad h. l.), taken no doubt from the land and transferred to persons or souls; so that we might supply Nephesh in this sense, "to destroy all, both those who have drunk its poison, and those also who are still thirsting for it" (Knobel). But even if we were to supply ארץ (the land), we should not have…

The first words of this chapter are the contents of it, "These are the words of the covenant" (Deu 29:1), that is, these that follow. Here is, I. A recital of God's dealings with them, in order to the bringing of them into this covenant (Deu 29:2-8). II. A solemn charge to them to keep the covenant (Deu 29:9). III. An abstract of the covenant itself (Deu 29:12, Deu 29:13). IV. A specification of the persons taken into the covenant (Deu 29:10, Deu 29:11, Deu 29:14, Deu 29:15). V. An intimation of the great design of this covenant against idolatry, in a parenthesis (Deu 29:16, Deu 29:17). VI. A most solemn and dreadful denunciation of the wrath of God against such persons as promise themselves peace in a sinful way (Deu 29:18-28). VII. The conclusion of this treaty, with a distinction between things secret and things revealed (Deu 29:29).

29:18 This metaphor describes idolatry (the root), which, if cultivated, would result in idol worship (the fruit).