8 For a sign that shall be spoken against. By a sign, we properly understand in Scripture a cross. Moses, it is said, set the serpent “upon a pole.” That is upon a cross. Or else a sign is indicative of something strange and obscure seen by the simple but understood by the intelligent. There is no cessation of controversy about the Incarnation of the Lord; some asserting that he assumed a body, and others that his sojourn was bodiless; some that he had a passible body, and others that he fulfilled the bodily œconomy by a kind of appearance. Some say that his body was earthly, some that it was heavenly; some that He pre-existed before the ages; some that He took His beginning from Mary. It is on this account that He is a sign that shall be spoken against.
Source: Letters (New Advent)