John Gill Commentary Deuteronomy 6:25

"And it shall be righteousness unto us, if we observe to do all this commandment before Jehovah our God, as he hath commanded us." — Deuteronomy 6:25 (ASV)

And it shall be our righteousness
Or a mercy, benefit, and blessing to us; or this shall be reckoned our righteousness, and that by which we shall be justified:

if we observe to do all these commandments before the Lord our God, as
he has commanded us ;
in order to have such a justifying righteousness, a man must keep all the commandments of God, not one excepted; and that perfectly, without the least breach of them in thought, word, or deed; and that before the Lord, in his sight, not as it may appear to a man himself, or to others, but as it appears to God, who sees the heart, and weighs all actions; and a man must keep them in the manner the Lord has commanded, even with all his heart, soul, and strength, as in (Deuteronomy 6:5) and this is not possible for a sinful man to do; and therefore righteousness cannot be by the law.

Only Christ could thus keep all the commandments of God, and his obedience is our righteousness; and he only is the end of the law for righteousness to everyone that believes, and to him we must seek for it.