Heidelberg Catechism

Question 62. But why can’t our good works be our righteousness before God, or at least part of it? 

Answer: Because the righteousness which can stand before God’s judgment must be absolutely perfect, (a)  In this life, even our best works are imperfect and defiled with sin. (b)

(Scriptural proofs after the fold.)

a. Galatians 3:10

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.”

      Deuteronomy 27:26

‘Cursed be anyone who does not confirm the words of this law by doing them.’ And all the people shall say, ‘Amen.’

b. Isaiah 64:6

We have all become like one who is unclean, and all our righteous deeds are like a polluted garment. We all fade like a leaf, and our iniquities, like the wind, take us away.