Deuteronomy 20:18

18 so that they won't teach you to do all the detestable things they do for their gods, and you sin against the Lord your God.[a]

Deuteronomy 20:18 Meaning and Commentary

Deuteronomy 20:18

That they teach you not to do after all their abominations,
&c.] This is another reason why they were to be utterly destroyed, not only because of the abominations which they committed, but to prevent the Israelites being taught by them to do the same; wherefore, as before observed from Jarchi, such as became proselytes were suffered to live among them, because there was no danger of idolatry from them, which even proselytes of the gate renounced; and though all other abominations are included, yet this is particularly respected, as appears from the following clause:

which they have done unto their gods;
to the honour of whom not only many superstitious rites and ceremonies were performed, and idolatrous actions committed, but acts of lewdness, and even unnatural uncleanness:

so should ye sin against the Lord your God;
a sin the most provoking to him, as the sin of idolatry was; and cause his anger to rise to such a degree, as to suffer them to be carried captive from the land he gave them to inherit; and which afterwards, was the case, and that through learning the manners and customs of these people; see ( Psalms 106:34-42 ) .

Deuteronomy 20:18 In-Context

16 However, you must not let any living thing survive among the cities of these people the Lord your God is giving you as an inheritance.
17 You must completely destroy them-the Hittite, Amorite, Canaanite, Perizzite, Hivite, and Jebusite-as the Lord your God has commanded you,
18 so that they won't teach you to do all the detestable things they do for their gods, and you sin against the Lord your God.
19 "When you lay siege to a city for a long time, fighting against it in order to capture it, you must not destroy its trees by putting an ax to them, because you can get food from them. You must not cut them down. Are trees of the field human, to come under siege by you?
20 But you may destroy the trees that you know do not produce food. You may cut them down to build siege works against the city that is waging war with you, until it falls.

Footnotes 1

Holman Christian Standard Bible ® Copyright © 2003, 2002, 2000, 1999 by Holman Bible Publishers.  Used by permission.  All rights reserved.