This can be stated in an active form for clarity. AT: "Then John's disciples and a Jew began to argue" (See: [[rc://en/ta/man/translate/figs-activepassive]])
In this phrase, "look" is a command meaning "pay attention!" AT: "you have testified, 'Look! He is baptizing,'" or "you have testified. 'Look at that! He is baptizing,'" (See: rc://en/ta/man/translate/figs-explicit)