Four verses, one commandment
In the Jewish count, everything from "no other gods" to "those who love Me" is the second statement.
Three things are forbidden here: having other gods, making images, and bowing down to them or serving them. In the Gemara, Rav Hamnuna counts this statement with the first as the two the people heard straight from God: "'I am' and 'You shall have no other gods' we heard from the mouth of the Almighty" (Makkot 24a; see I Am Hashem Your God).
The Rambam (Rabbi Moshe ben Maimon, Egypt, 1100s) shows how far it reaches. It is not only about what you do. It is about what you let into your mind:
So how did people ever come to bow down to stars and statues? The Rambam opens his Laws of Idolatry with the answer, told as history.
How the world forgot, and how it remembered
From Enosh, Adam's grandson, to Moshe. It does not start with people denying God. It starts with an argument that sounded religious. Step through it.
The Rambam turns this history into a rule. Serving a star, an angel or anything else that was created is idolatry, even for someone who knows that Hashem is God, if he serves it the way Enosh's generation did:
Avraham in the idol shop
The Torah does not tell this story. A midrash does, to explain one short verse. Step through it.
The verse is Bereishit 11:28: "Haran died in the presence of Terach his father, in the land of his birth, in Ur Kasdim." Why say "in the presence of his father"? Bereishit Rabbah (a collection of the Sages' teachings on Bereishit) answers with a story told by Rabbi Chiya, the grandson of Rav Ada of Yafo.
Our own observation: the story and the Rambam's history meet at the same point: a person who thinks for himself, in a house full of idols. For more of Avram's story, see Avram's Story.
The third and fourth generation
Does God punish children for what their parents did?
It sounds unfair, and another verse seems to say the opposite:
The Gemara sets the words "He visits the sin of fathers on children" (in our verse, and again in Shemot 34:7) against Devarim 24:16. It does this while asking why some good people suffer, and it uses the answer to rule out one explanation: that a good person suffers for a wicked father's sins.
The Gemara gives the same answer in Sanhedrin 27b. Rashi (the classic commentator on the Torah and the Gemara, France, 1000s) finds it inside our verse, in its last words, "of those who hate Me." He points to the Aramaic translation of Onkelos:
Our own observation, in plain words: the verse is about children who carry on what their parents did wrong. A child who leaves it is judged as himself. And the next verse sets the two sides next to each other: the sin reaches the third and fourth generation of those who keep it going, and the kindness reaches "thousands" of those who love Hashem.
Its partner on the other tablet
One view in the Mechilta, an early midrash, reads the two tablets side by side. In that view, across from "no other gods" stands "Do not commit adultery."
The Mechilta (an early midrash on Shemot, from the time of the Mishnah) calls God HaMakom, "the Place," and its proof is a verse from the prophet Yechezkel (16:32). It goes on to quote Hoshea (3:1) as well.
Our own observation, in plain words: the prophets describe the bond between Hashem and Israel as a marriage, and Israel's idolatry as a wife who betrays her husband. Serving another god is not a private opinion about religion. It is unfaithfulness inside a covenant. That fits the reason the Mechilta gives for setting the commandment about loyalty to God across from the commandment about loyalty in a marriage.
This is the view of Rabbi Chananya ben Gamliel. The same passage ends with a second opinion:
Where it stops
What the second commandment does not reach, where it needs a rav, and the one place it never bends.
Not on a child who leaves it
"The third and fourth generation" applies to children who hold on to their fathers' deeds. A child who leaves them is not punished for them (Berachot 7a; Sanhedrin 27b; see above).
Images: ask your rav
The commandment also forbids making images, and a later verse at Sinai repeats it:
The Rambam learns from it that some images are forbidden even as decoration:
In the Rambam's rules (Laws of Idolatry 3:10-11): a human figure that stands out from the surface, a statue or a carving in relief, is forbidden even as decoration. A sunken, painted or woven human picture is permitted. Images of the sun, the moon, the stars, the constellations and the angels are forbidden even when flat. Animals, trees and plants are allowed, even raised.
The details are delicate. Statues, dolls, action figures, a 3D print, a ring with a face: how these rules apply to them today is discussed at length by the poskim (rabbis who decide Jewish law), and they do not all agree. Before you make or keep one, ask your rav.
The Shulchan Aruch (Yoreh De'ah 141:4-7) adds rules that the Rambam does not give here, about keeping such images and about partial figures.
The line that never bends
Almost any mitzvah gives way to save a life. Idolatry is one of three that do not:
Rabbi Yochanan reports this in the name of Rabbi Shimon ben Yehotzadak, as a vote the Sages took in an upper room in Lod. The Gemara there adds cases where a person must give his life even for a smaller mitzvah, and real cases are never ones to decide alone.
Common questions
Short answers, with sources.
What does "lo yihyeh" mean?
"There shall not be," the opening words of Shemot 20:3: "You shall have no other gods before Me." In the Jewish count, verses 3 to 6 are one statement, the second of the ten: no other gods, no images, and no bowing down to them or serving them.
What counts as avodah zarah?
Avodah zarah means "foreign worship," idolatry. The Rambam defines its core as worshipping any created thing, an angel, a star or anything else, the way Enosh's generation did, even if the person knows that Hashem is God (Laws of Idolatry 2:1).
Did the first idol worshippers deny God?
Not at first, says the Rambam. In the days of Enosh, people reasoned that honoring the stars, God's servants, was a way to honor God. The images and the false prophets came later, and only then was God forgotten (Laws of Idolatry 1:1-2).
How old was Avraham when he found God?
The Rambam says Avraham began wondering as a small child and recognized his Creator at forty (Laws of Idolatry 1:3). In the Gemara, Rabbi Ami bar Abba says three (Nedarim 32a), and Bereishit Rabbah 64:4 gives both three and forty-eight. The Kesef Mishneh brings a way to hold both: at three he began, and at forty he came to know his Creator fully.
Is the idol shop story in the Torah?
No. It is a midrash in Bereishit Rabbah 38:13, told to explain why the Torah says that Haran died "in the presence of Terach his father" (Bereishit 11:28).
Are children punished for their parents' sins?
Only when they hold on to their parents' deeds. The Gemara uses this to settle Shemot 20:5 with Devarim 24:16 (Berachot 7a; Sanhedrin 27b), and Rashi reads it in the verse's own words, "of those who hate Me" (Rashi on Shemot 20:5).
Can I keep a statue, a doll or a photo?
The Rambam forbids making a raised human figure even for decoration and allows a flat picture (Laws of Idolatry 3:10-11). Whether you may make or keep a doll, a statue or a photo today is a question for your rav.
Why does the Mechilta pair "no other gods" with "do not commit adultery"?
In Rabbi Chananya ben Gamliel's view, with five statements on each tablet, the second stands across from the seventh, to teach that whoever worships idols is counted as if he committed adultery behind God's back (Mechilta d'Rabbi Yishmael, Bachodesh 8). The Sages there say all ten were written on each tablet.
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