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

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.