Hebrews 10:1
5 “For” (he says) “the Law having a shadow of the good things to come, not the very image of the things”; i.e. not the very reality. For as in painting, so long as one [only] draws the outlines, it is a sort of “shadow” but when one has added the bright paints and laid in the colors, then it becomes “an image.” Something of this kind also was the Law.
“For” (he says) “the Law having a shadow of the good things to come, not the very image of the things,” i.e. of the sacrifice, of the remission: “can never by those sacrifices with which they offered continually make the comers thereunto perfect.”
Source: Homilies on the Epistle to the Hebrews (New Advent)