But why cannot our good works be the whole or part of our righteousness before God?

Heidelberg Catechism

Question 62. But why cannot our good works be the whole or part of our righteousness before God?

Answer. Because the righteousness which can stand before the judgment seat of God, must be perfect throughout and entirely conformable to the divine law,[1] but even our best works in this life are all imperfect and defiled with sin.[2]

[1] Deut 27:26; Gal 3:10;
[2] Isa 64:6; Php 3:12; Jas 2:10