“All of us have become like one who is unclean, and all our righteous acts are like filthy rags; we all shrivel up like a leaf, and like the wind our sins sweep us away” (Isaiah 64:6). This passage is often used as a proof text to condemn all our acts of goodness as nothing more than “filthy rags” in the eyes of God. The context of this passage is referring specifically to the Israelites in Isaiah’s time (760—670 B.C.) who had strayed from God. Isaiah was writing concerning his nation and their hypocrisy. Yet he includeshimselfin the description, saying “we” and “our.” Isaiah was redeemed and set apart as a prophet of God, yet he saw himself as part of a group that was utterly sinful. The doctrine of total depravity is taught clearly elsewhere in Scripture (e.g.,Ephesians 2:1–5), and the illustration of Isaiah 64:6 could rightly be applied to the whole world, especially given Isaiah's inclusion of himself in the description.
The term “filthy rags” is quite strong. The word filthy is a translation of the Hebrew word iddah, which literally means “the bodily fluids from a woman’s menstrual cycle.” The word rags is a translation of begged, meaning “a rag or garment.” Therefore, these “righteous acts” are considered by God as repugnant as a soiled feminine hygiene product.
Read more: http://www.gotquestions.org/filthy-rags.html