וַיֹּ֤אמֶר לּוֹ֙ מַלְאַ֣ךְ יְהוָ֔ה לָ֥מָּה זֶּ֖ה תִּשְׁאַ֣ל לִשְׁמִ֑י וְהוּא־פֶ֛לִאי׃ (ס)
永恆者的天使對他說：«你為甚麼問我的名字呢？我的名字是隱祕的。»
Rashi
When it is mystical. It is concealed. It changes constantly,4According to the angel’s mission at that particular time. Cf. Rashi to Bereishis, 32:30. so that its identity remains unknown on any given day.