Why does it say "ki'Devar Hashem"?
Malbim (1-2): He did like all the details that Hashem commanded about it; there were seven. (a) Go constantly to buy a belt - this will make an impression. (b) Buy it for money. (c) Buy it according to your size. (d) It should be an Ezor (which is specifically for the loins), and not a Chagor (which can be tied around the body anywhere from the heart to the legs). (e) It should be of linen, a sign of Kehunah and being Nasi. (f) It should be on your loins constantly. He did not say "v'Azarto", for one cannot constantly gird tightly. (g) Do not put it in water.
What is the significance of the commands about the belt?
Malbim (1): Verse 11 explains that sticking the belt to Yirmeyah's loins is a metaphor for Hashem clinging Yisrael to Himself; everything corresponds. (a) Hashem 'went' many times to acquire Yisrael - he sent [Moshe to Pharaoh] many times to redeem them from Egypt, and increased signs and wonders. (b) He bought it for money - He did for them many signs and wonders, and changed the order of creation (nature) 1 . (c) He bought them according to His size - "vi'Hyisem Kedoshim Ki Kadosh Ani" (Vayikra 11:44), "v'Halachta bi'Drachav" (Devarim 28:9), as if [Yisrael] are a garment correspond to His Midos. Through them He shows His conduct and Chachmah, which are called garments. (d) They are compared to an Ezor. After the Kinyan he wore them, i.e. His deeds are revealed through them. Just like an Ezor clings to the loins, so they cling to Him to directly supervise them Hashgachah Pratis, not via an intermediary. An Ezor symbolizes strength. (e) Linen symbolizes honor and glory. So He stuck Yisrael for praise, fame and glory; via them, His Gevurah is seen, that He changes the Ma'arachah, and His glory in acts of Hashgachah. (f) It should be on your loins constantly - like Divine clinging is constant. Also when He acts via nature, and does not gird with Gevurah, [the belt] is on His loins and He is prepared [to change nature] constantly. (g) A linen garment must be laundered constantly in water - so Yisrael must constantly cleanse themselves from evil via water, i.e. hearing Torah and Nevu'ah - "Kabsi mi'Ra'ah Libech" (4:14). Not only did they not want to be laundered, to do Mitzvos Hashem - they did not even want to be put in water, i.e. to hear words of Musar.
Malbim: It is as if He spent for them His money and His other acquisitions, i.e. conducting nature and its laws. This is Hashem's Kinyan in His world "Koneh Shamayim va'Aretz" (Bereishis 14:19). He changed the order for His nation.
Did Yirmeyah truly buy the belt [and do everything written here]?
Radak (1): Yes.
Radak (1, citing the Rambam): No. All this was in a prophetic dream.
