Jesus asks this question to remind the man that no man is good the way God is good. Alternate translation: "You do not understand what you are saying when you call me good." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This double negative emphasizes that God is the only one who is good. Alternate translation: "The only one who is good is God" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])