Hebrews 7:18
“For there is verily” (he says) “a disannulling of the commandment going before, for the weakness and unprofitableness thereof.” Here the Heretics press on. But listen attentively. He did not say “for the evil,” nor, “for the viciousness,” but “for the weakness and unprofitableness [thereof],” yea and in other places also he shows the weakness; as when he says “In that it was weak through the flesh.” [The law] itself then is not weak, but we.
Source: Homilies on the Epistle to the Hebrews (New Advent)