“Good deeds are worthless as a filthy rag in the sight of God not because the motive of the doer is necessarily wrong, but because the debt of sin owed is too great to be overlooked on account of such deeds.” (Leighton Flowers, according to this source)
Well, actually, remember that this statement is made in Isaiah. Sure its in one of the later chapters, 65 is it? (No, its actually 64). But still the opening in chapter 1 is part of the context. Isaiah’s generation sought righteousness purely by ceremonial means, i.e. sacrifices. They committed adultery, fornication, murder, whatever they wanted, and then said, “its ok, because I offer this goat.” And he tells them in chapter 1 to “cleanse your hands ye sinners and purify your hearts ye double minded; cease doing evil, learn to do well” i.e. to live morally and pursue a righteousness based on living morally not just on ceremonial stuff.
It is with that context (i.e. of the generation he is speaking to) in mind that he says: All OUR righteousness is as filthy rags.
OUR means his generation. All their righteousness was filthy rags because it was purely a ceremonial righteousness, purely based on sacrifice and not on living morally. Ironically, people who believe in OSAS also pursue a righteousness based on sacrifice alone and refuse to live morally. Just because that sacrifice is now Jesus’ sacrifice rather than a goat doesn’t change anything; Isaiah would still say to them “cleanse your hands ye sinners and purify your hearts ye double minded; cease doing evil, learn to do well” i.e. to live morally and pursue a righteousness based on living morally not just on a sacrifice.