Q:  But why cannot our good works be the whole, or part of our righteousness before God?
A:  Because, that the righteousness, which can be approved of before the tribunal of God, must be absolutely perfect,¹ and in all respects conformable to the divine law; and also, that our best works in this life are all imperfect and defiled with sin.²

–The Heidelberg Catechism (#62)

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¹ “For all who rely on works of the law are under a curse; for it is written, “Cursed be everyone who does not abide by all things written in the Book of the Law, and do them.”” (Galatians 3:10 ESV); ““‘Cursed be anyone who does not confirm the words of this law by doing them.’ And all the people shall say, ‘Amen.’” (Deuteronomy 27:26 ESV)
² “We have all become like one who is unclean, and all our righteous deeds are like a polluted garment.” (Isaiah 64:6 ESV)