Joseph, unlike his ancestors, delights in the gods of the gentes: he knows their names and attributes. He thinks about them, he plays in his mind with those names and attributes; he can’t help himself. When Jacob comes upon his son in nude contemplation he thinks Joseph is “blowing kisses to the stars,” which the lad denies, but in a flood of verbiage — he is an incessant chatterbox and will one day pay mightily for it — that takes him right back into danger. He soon finds himself describing the worship of the Mesopotamian moon-god Sin, whose “day of festive contemplation,” Shapattu, is coming soon, and recalls that the moon does not shine on its own, no, we know that “He made it to shine” and —
“Who?” Jacob asked softly. “Who made it shine?”
“Marduk-Baal!” Joseph cried all too hastily, but followed this at once with a long, drawn-out “Aeh-h-h-h,” shaking his head to undo it, and now continued, “… as He is called in the old tales. It is, however — as my dear papa has no need to learn from his poor child — the Lord of the gods, who is stronger than all the Annunaki and Baals of other nations, the god of Abraham, who defeated the dragon and created the threefold world.” (76)
Joseph is this, if not consciously and intentionally polytheistic, imaginatively so; moreover, he is, even when speaking conciliatory words to his father, not a strict monotheist but rather a henotheist — which suggests that that he thinks this may be acceptable to his father. (Mann is surely aware of the passages in the Hebrew Bible that sound henotheistic, for instance Psalm 95:3: “For the LORD is a great God, and a great King above all gods.”) But we readers know that it is not so acceptable. In this very chapter Mann describes a conversation Jacob had on just this subject with a man named Jebshe:
If the God who had established the sun, the signs along its path, and the planets, including the earth, was the highest God, then he was also the only god, and it would be best not even to speak of other gods, in such a case, otherwise one would be forced to label them with the name Jacob had refrain from using, precisely because reason demanded that the term and concept of “the highest God” be equated with the only God. 
Jacob is horrified by any suggestion that the gods of the gentes are to be treated with anything but contempt and revulsion, and Joseph has to employ his best and most charming eloquence to calm his father’s troubled spirit. (It is, fortunately for him, a task he is always up to.)
So this opening scene of the story-as-such establishes this tension between the single-minded devotion of Jacob to the Fear and Joseph’s playful delight in contemplating the religions of the gentes. It wouldn’t be right to say that Joseph simply is polytheistic. But he is inclined to enjoy correspondences and to seek whenever possible a reconciliation of opposing forces. The tetralogy as a whole is called Joseph and His Brothers, but I think in a more fundamental sense it’s about Joseph and his father. It explores the difference between a radical uncompromising monotheism and a more … flexible approach to matters of faith.