Tafsir of Az-Zumar 39:45

"And when Allah is mentioned alone..." The pivot of the meaning rests upon the word "alone." That is: when Allah is singled out in mention, and their gods are not mentioned alongside Him, they become disgusted—meaning they recoil and contract.

"And when those [others] besides Him are mentioned..." Whether their gods are mentioned alongside Allah or mentioned alone, they rejoice. This is due to their infatuation with them and their neglect of the right of Allah, in favor of their own desires regarding them.

It is said: When it is said, "There is no god but Allah alone, with no partner," they recoil, because it contains the negation of their gods.

It is also said: It refers to their rejoicing at what the tongue of the Messenger of Allah (ﷺ) had previously uttered regarding the mention of their gods when he recited Surat an-Najm at the door of the Kaaba, causing them to prostrate with him out of joy.

Rejoicing and disgust are set in opposition here, as each is the extreme of its kind:

• Rejoicing: That the heart is filled with such happiness that the skin of the face expands and beams.
• Disgust: That the heart is filled with such grief and rage that the contraction appears on the surface of the face.

If you ask: What is the operative factor (‘amil) in "when they are mentioned"? I say: The operative factor is the "when" of suddenness (idha al-mufaja’ah). The estimation is: "At the time those besides Him are mentioned, they suddenly break into rejoicing."

"Say, 'O Allah, Creator of the heavens and the earth, Knower of the unseen and the witnessed, You will judge between Your servants concerning that over which they used to differ.'"