If we say that we haven't sinned, we make him a liar, and his word is not in us.
His word is not in us ... Characteristic of John's writings is his use of such words as "word" and "truth" to stand for the whole body of gospel teaching.  Moreover, "the truth" or "the word" in John's view was not some indefinite and nebulous goal pursued by the Christians seeking to know it; it was something which they already knew, already had, already walked in, already obeyed.  The reference, of course, is to the basic gospel of the New Testament which is perfect, complete, final, and sufficient.  It is not to be added to nor taken from.  Such is the Johannine conception of the message which he and other apostles delivered to people that they might be saved.