Why our good works cannot be part of our righteousness before God.

Heidelberg Catechism (extended)

The Catechism
Method of Instruction in the Christian Religion
As the Same is Taught in the Reformed Churches and Schools

(with the Scripture references written out)

Note. This Catechism is fully based on the Scriptures. The references to Scripture are indicated in parentheses with a letter. For example, the letter (a) points to the texts (a) placed after the answer.

24. Lord's Day 

 Q. 62. 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, (a) 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. (b) 

 (a) Gal.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. Deut.27:26 Cursed be he that confirmeth not all the words of this law to do them. And all the people shall say, Amen. 

(b) Isa.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. 

 Q. 63. What! do not our good works merit, which yet God will reward in this and in a future life? 

A. This reward is not of merit, but of grace. (a) 

 (a) Luke 17:10 So likewise ye, when ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do. 

 Q. 64. But does not this doctrine make men careless and profane? 

A. By no means: for it is impossible that those, who are implanted into Christ by a true faith, should not bring forth fruits of thankfulness. (a) 

 (a) Matt.7:18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. John 15:5 I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.

Source: http://www.ccel.org/creeds/heidelberg-cat-ext.txt