A.T. Robertson Commentary 1 John 3:18

"[My] Little children, let us not love in word, neither with the tongue; but in deed and truth." — 1 John 3:18 (ASV)

In word, neither with the tongue (λογω μηδε τη γλωσση). 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" (εν εργω κα αληθεια). Here is a case where actions do speak louder than mere words.