Q. BS”D, Dear Rabbi Bartfeld, Regarding toveling of dishes, utensils, I see nowhere in the Torah, midrash, tanach etc. to tovel dishes, I think that this is strictly a rabbinic law.
Can you please answer why we have to tovel dishes?

A. The Talmud (Avoda Zara 75b) deduces the mitzva of immersing utensils used for food consumption from the posuk on Parshas Matos (Bamidbar 31: 23) relating to the booty taken from the Midianites; “Everything that comes into the fire, you shall pass through fire and it will become pure”, Though the addition of the redundant word “Vetaher” the Torah added another type of purification besides purging (koshering from prohibited foods imbedded in the utensil), namely tevila. The Talmud presents there an additional source for tevilas kelim, namely from the words: “B’mei nida ischata,” (ibid.) and explains the need for the two sources. The obligation to immerse kelim in a mikva is recorded by the Rambam (M’aacholos Assuros 17: 3, Shulchan Aruch Y.D. 120 : 1, and in virtually any Halacha sefer dealing on the topic.
Most Poskim maintain that the mitzvah is biblical, at least for metal utensils, since it is inferred from the above verse. (Rashi Avoda Zara ibid., Tosafot ibid. Smak 199, Or Zarua 288 – 293, Raved, Ramban, Rashbo, Ritva on Talmud ibid, et. al.) Some Poskim, however, opine that tevilas kelim is only Rabbinical (Rambam ibid., Tosafot Rid on Talmud above, Orchos chaim 2: p. 154 et. al.)
Talmud Yerushalmi (Avoda Zara 5: 15) explains that the reason for the immersion of utensils acquired from Gentiles is to spiritually elevate the items originated in a domain that does not observe the mitzvos or maintains the holiness of the Torah, to a realm that does. Ritva (Avoda Zara 75b) quoting Ramban, compares the process as similar to the conversion of a ger to Judaism, it also demands tevila.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a