Why It Is Impossible To (Directly) Follow the Law

As I indicated here, a person is in fact justified by faith and not by any deed of righteousness he may do. There is a practical reason for this: because it is impossible to follow the law directly in a successful manner (Romans 9:30-33). Note also the following scripture:

Romans 7

18 I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out.
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22 For in my inner being I delight in God's law;
23 but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members.

The scripture above indicates that though someone who has faith may want to do good deeds directly, it is impossible for him to do so, because his sinful nature prevents this from happening. The question then arises, "How does someone go about doing good deeds?" The answer is, by having faith. When someone has faith, that person receives the Holy Spirit (Galatians 3:14), and the Holy Spirit destroys the deeds that person is compelled to do because of his sinful nature:

Romans 8

13 For if you live according to the sinful nature, you will die; but if by the Spirit you put to death the misdeeds of the body, you will live,

Someone may next ask, how does all of this work? When someone has faith, that person winds up being controlled by the Spirit (Romans 8:9). However, the person is also controlled by his sinful nature at the same time (Romans 7:24-25). A conflict actually arises between the (Holy) Spirit and the person's sinful nature (Galatians 5:17), with the Spirit increasingly destroying or eliminating the deeds caused by that person's sinful nature, as that person continues to have faith (Romans 8:13, 2 Corinthians 3:18). A very important point of all of this, is that a person cannot do good deeds or the deeds attributed to the Spirit directly. A person has to be compelled or forced by the Spirit into doing good deeds - given the fact that a person becomes a slave of God's law when he has the Spirit in him:

Romans 7

24 What a wretched man I am! Who will rescue me from this body of death?
25 Thanks be to God–through Jesus Christ our Lord! So then, I myself in my mind am a slave to God's law, but in the sinful nature a slave to the law of sin.

Therefore a person really has only two choices. He can decline to have faith, and be entirely controlled by his sinful nature - which compels or forces him to act sinfully. Or a person can have faith, and have the Spirit enter him, who increasingly compels or forces him to act righteously. A person never has direct, ultimate control over his actions (though it may seem that way at times). A person only has the choice of having faith and having the Spirit control him, or not having faith and having his sinful nature control him. (In the first case, the person behaves increasingly righteously in a natural way. In the second case, the person behaves unrighteously in a natural way.)

The above is why it is impossible for someone to achieve righteousness by directly pursuing it / the law, and why the only way a person can achieve righteousness is by having faith.