This is the name of God that he revealed to his people in the Old Testament. See the translationWord page about Yahweh concerning how to translate this.
This rhetorical question is asked as a rebuke for consulting Baal-Zebub. This may be written as a statement. This is irony because they are definitely aware of the God of Israel. Alternate translation: "You fools! You know there is a God in Israel, but you were acting as though you did not know when you sent men to consult Baal-Zebub, the god of Ekron!" (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-irony]])