What are the two evils?
Rashi: To switch the Power they fear for something similar is evil. However, they abandoned Me - a source of flowing water - to go after idolatry, which is like pits of gathered water, and they are broken and cracked, and the water is absorbed in the cracks. This is a second evil.
Malbim: (a) Abandoning Me was a double evil - "ha'Heimir Goy Elohim" (verse 11), and Yisrael abandoned a source of flowing water. (b) Switching Me for something non-existent (idolatry), and they needed to create it.
Why is Hashem compared to a source of flowing water?
Radak: This refers to the good influence that Hashem brings on Yisrael, when they guard the Torah. There are two benefits. (a) The water does not come from elsewhere, rather, from that place itself. (b) It does not cease.
Malbim: Hashem salvation is compared to a spring that flows by itself. People need not dig it. The more one draws from it, more water is added - "u'She'avtem Mayim b'Sason" (Yeshayah 12:3). It never ceases, and it is the first influencer that influences directly, not via something else.
What are "Be'eros"?
Radak: They are built receptacles to hold rainwater.
Why do they not hold water?
Rashi: The water erodes the brim [of the cisterns] and the walls, and they fall.
What is the comparison to broken cisterns?
Radak: If they are broken, the water leaves just like it enters. Yisrael relied on other gods and nations that served them, e.g. Melech Ashur and Melech Mitzrayim, and hoped that they would save them from their enemies. Even if they are whole, they cease when the water inside runs out, for the water comes from elsewhere. A king acts only with the help of his army, and also this helps only with Hashem's help - "Ein ha'Melech Nosha b'Rov Chayil" (Tehilim 33:16). If they trust in their gods to help them, these are like broken cisterns that do not hold water, for even if one puts water in them, it will leave between the shards, and he will not find water in it. So those who trust in other gods, even if they have many [soldiers], and weapons, it is as if they are not, and their enemies will defeat them, even if [their enemies] are stabbed 1 , and their grain and wine 2 will be cursed - "Pano El Harbeh v'Hinei li'M'at", "v'Haveisem ha'Bayis v'Nafachti Vo" (Chagai 1:9).
Malbim: (a) Great work is needed to dig them - "Masa Bahamos Negev b'Eretz Tzarah v'Tzukah" (Yeshayah 30:6). (b) One needs to put water in them - so they needed to give gifts and bribes [to the other kings] - "Yis'u Al Kesef Ayarim Cheilehem" (ibid.) (c) The water entered does not last. The other nations did not help them - "u'Mitzrayim Hevel va'Rik Ya'azoru" (ibid., 7).
