The reasons which led to the admission and retention of Qoheleth-Ecclesiastes in the Hebrew Bible and, consequently, in the Christian canon, were probably these. In the first place, its heterodox statements are balanced by others of unimpeachable orthodoxy, such as ii26, iii17, and viii12-13. Some of these are, in fact, quotations which Qoheleth includes only to refute them. In other cases, they sound pious, but really are part and parcel of his thought. Some, such as xi9c, may be marginal notations by a later hand…. Nevertheless, the daring originality of Qoheleth’s thought and its challenge to traditional teachings could not be concealed, as the continuing objections to the work bear witness. …
A second reason for the acceptance of Qoheleth was the unquestionable intellectual and literary quality of the book, to say nothing of its ethical wisdom….
R.B.Y. Scott, Proverbs and Ecclesiastes (Garden City, NY: Doubleday & Company, 1965), 194.