Or HaChaim on Genesis 35:2/אור החיים על בראשית ל״ה:ב׳

1 הסירו את אלוהי הנכר, "Remove the foreign gods!" Seeing that Jacob was about to leave the present location he was afraid that perhaps someone would take along trinkets which had served the people of Shechem as idols. Since Jacob's entourage did not include idol worshippers of the local population there had not been anyone who could neutralise these gods before they had come into the tents of Jacob's camp. Once that happened there is no way a Jew can neutralise these gods except by destroying them completely.

2Avodah Zarah 43 teaches that when one forces a Gentile to renounce his idol or even that belonging to a friend of his, such renunciation is legally valid. Once the idol has become the possession of a Jew, the only way it can be neutralised is by utterly destroying it. The Talmud distinguishes between adult Gentiles and children, however. The reason Jacob or the brothers did not force the surviving women of Shechem to neutralise these gods may have been that those women were halachically in the same category as children, i.e. that their actions had no validity in law. It is also possible that the sons of Jacob had simply not been careful to differentiate between different kinds of the loot. When they examined it at home it was already too late to do anything about neutralising these objects and making them usable for a Jew. Some of these idols may even have been hidden within other larger vessels which the sons of Jacob had captured. Another possibility is that the gods which had been neutralised in time somehow became mixed with others which had not.

3Jacob instructed the members of his household to purify themselves because contact with idols defiles a person. Maimonides writes in Hilchot Avot Ha-Tumah chapter six that any אב הטומאה, source of serious impurity, which confers impurity through touch or through carrying same, not only confers impurity on any person who touches same but also on anyone touching or carrying objects that have been in touch with the person who is אב הטומאה. Accordingly, Jacob was quite right when he instructed his people to purify themselves and their clothing even if not they personally, but only their clothing had been in contact with the idolatrous trinket. Jacob was also very precise in referring to שמלותיכם, your garments. He meant that the garments worn by the people of Shechem at the time they were slain had to be purified also.

4You may ask that there should have been no need to change clothing seeing that clothing could have become defiled only through direct contact with the idolatrous object, an אב הטומאה, and even if these garments had been in direct contact with the idolatrous object they could not have conferred impurity on a human being seeing they themselves were not primary sources of impurity. When we read in Psalms 106,28: ויאכלו מזבחי מתים, "they ate sacrifices offered to the dead," the word "dead" is understood to be a reference to idolatry. Idolatry and the dead are equated halachically for the purpose of the transference of impurity. Chulin 3 states that the sword of a slain person is of the same degree of impurity as the person who has been slain with it. [normally it would be of a lesser degree, seeing it is not the original source of that impurity, Ed.] According to Maimonides Hilchot Tum-at met chapter chapter 5 the example of "sword" is applicable to any of the objects that were attached to the slain person at the time he died, including his garments. According to the foregoing the garments of the people of Shechem then were capable of transferring impurity to human beings just as the sword which kills a person.