Heidelberg Catechism · 1563

Q62. But why can our good works not be our righteousness before God, or at least a part of it?

Because the righteousness which can stand before God's judgment must be absolutely perfect and in complete agreement with the law of God, whereas even our best works in this life are all imperfect and defiled with sin.
Answer · Heidelberg Catechism

Scripture proofs

  1. [1]
    Deuteronomy 27:26

    Cursed be he that confirmeth not all the words of this law to do them. And all the people shall say, Amen.

    Galatians 3:10

    For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them.

  2. [2]
    Isaiah 64:6

    But we are all as an unclean thing, and all our righteousnesses are as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.

Proof texts as given in the original; verse text from the King James Version.