In verse 18 there is a break from the main story line as John provides background information about the Jews' disbelief. (See: [[rc://en/ta/man/translate/writing-background]])
This remark appears in the form of a question to emphasize that Jesus' signs prove he is not a sinner. Alternate translation: "A sinner can not do such signs!" (See: [[rc://en/ta/man/translate/figs-rquestion]])