In word, neither with the tongue (logwi mhde th glwssh). Either instrumental or locative makes sense. What John means is "not merely by word or by the tongue." He does not condemn kind words which are comforting and cheering, but warm words should be accompanied by warm deeds to make real "in deed and in truth" (en ergwi kai alhqeiai). Here is a case where actions do speak louder than mere words.