FOR MOSES WRITES THAT THE MAN WHO PRACTICES THE RIGHTEOUSNESS WHICH IS BASED ON LAW SHALL LIVE BY THAT RIGHTEOUSNESS: Mouses gar graphei (3SPAI) ten dikaiosunen ten ek (tou) nomou hoti o poiesas (AAPMSN) auta anthropos zesetai (3SFMI) en autois: (Nehemiah 9:29; Ezekiel 20:11,13,21; Luke 10:27,28; Galatians 3:12)

This quotation is actually not Moses’ words but God’s word to and through Moses. Paul quoted from the Old Testament to prove to any Jewish readers that they did not even understand their own Law.

Paul began with

'So you shall keep My statutes and My judgments, by which a man may live if he does them; I am the LORD. (Leviticus 18:5)

This verse states the purpose of the Law: if you obey it, you live.  The religious Jew would argue

“But we did obey it!”

To which Paul would respond...

"You may have obeyed it outwardly, but you did not believe it from your heart.” (see notes on Romans 2:28-29)

The Mosaic Law makes the path to righteousness through the law plain. If you want to live by the law (find life through the law), you must do the law - and do it completely and perfectly. The Amplified Version accentuates this demand...

For Moses writes that the man who [can] practice the righteousness (perfect conformity to God's will) which is based on the Law [with all its intricate demands] shall live by it.

One Jewish interpretation of Leviticus 18:5 was that those who keep the commandments merit (earn) eternal life. This misinterpretation of the passage appears in Jewish texts alongside the view that God elects Israel as a whole to be saved.

The verse in fact says that if (an impossible "if") one could keep the law in its entirety, he would be acceptable to God, but the Law must be taken in whole or not at all...

For whoever keeps the whole law and yet stumbles in one point, he has become guilty of all. (James 2:10)

In Ezekiel we read a similar teaching...

"And I gave them My statutes and informed them of My ordinances, by which, if a man observes them, he will live... 13 But the house of Israel rebelled against Me in the wilderness. They did not walk in My statutes, and they rejected My ordinances, by which, if a man observes them, he will live; and My sabbaths they greatly profaned. Then I resolved to pour out My wrath on them in the wilderness, to annihilate them... 21 "But the children rebelled against Me; they did not walk in My statutes, nor were they careful to observe My ordinances, by which, if a man observes them, he will live; they profaned My sabbaths. So I resolved to pour out My wrath on them, to accomplish My anger against them in the wilderness. (Ezekiel 20:11,13,21)

James and Ezekiel remind us that if it were possible to keep the law even for a while, a person who failed in only one point of the law would remain just as lost and under God's wrath as a person who failed in every point of the law.

Anyone who is not utterly self-deceived realizes the impossibility of never stumbling even in the smallest way. And the foolish person who does presumptuously rely on his own obedience to the law will attain only the imperfect and unacceptable righteousness that his imperfect obedience merits. In God’s sight, such righteousness is wholly unrighteous and can never remove sin or earn divine favor. Because of the countless rabbinical traditions that had been developed over the previous several hundred years, the Jews of Paul’s time had so lowered and replaced with manmade traditions, God’s divine standard of righteousness, that many Jews actually believed they lived in satisfactory obedience to the law. Witness for example the rich young ruler, who after Jesus citation of several Old Testament commandments, responded with what seems to be a sincere reply...

“All these things I have kept” (Mt 19:20).

Only Christ fulfilled "all these things" in the Law perfectly (cf Mt 5:17).

To repeat, whoever relies on his own obedience to the Law is held accountable for everything that the Law requires. The righteousness which is based on law demands absolute perfection in every detail of the law.

The truths that Paul emphasizes here may be summarized as...

First, the man who pursues salvation by trying to keep the law will be judged on the basis of that effort.

Second, it is impossible to keep all the law.

Third, the inevitable failure of works righteousness results in eternal damnation.

But the righteousness based on faith [imputed by God and bringing right relationship with Him] (Amplified)

Paul contrasts Moses statement in (Leviticus 18:5) with the righteousness based on faith (here personified as if able to speak) which says another thing.

It is clearly implied in the previous verse that the attainment of justification by a method which prescribed perfect obedience is impossible for sinful men. It is the purpose of this and the succeeding verses to declare that the Gospel requires no such impossibilities. It neither requires us to scale the heavens, nor to fathom the great abyss. It demands only faith.

DO NOT SAY IN YOUR HEART WHO WILL ASCEND INTO HEAVEN?' (THAT IS, TO BRING CHRIST DOWN): me eipes (2SAAS) en te kardia sou tis anabesetai (3SFMI) eis ton ouranon; tout estin (3SPAI) Christon katagagein (AAN): (Proverbs 30:4)  (John 3:12,13; 6:33,38,50,51,58; Ephesians 4:8-10; Hebrews 1:3)

Paul's will now show that the knowledge of the will of God had been made perfectly accessible and that no one was ever required to do what was impossible in order to attain it. This knowledge was neither hidden, nor far off, but was obvious and at hand. To prove his point Paul appeals again to the Old Testament where we read that...

"For this commandment which I command you today is not too difficult for you, nor is it out of reach. 12 "It is not in heaven, that you should say, 'Who will go up to heaven for us to get it for us and make us hear it, that we may observe it?' 13 "Nor is it beyond the sea, that you should say, 'Who will cross the sea for us to get it for us and make us hear it, that we may observe it?' 14 "But the word is very near you, in your mouth and in your heart, that you may observe it. (Deuteronomy 30:11-14)

Note that here Romans Paul gives us the spiritual understanding of Moses' admonition in (Deuteronomy 30:11-14). Thus Paul saw “the commandment” or “the Word” (John 1:1,17 writes that "In the beginning was the Word, and the Word was with God, and the Word was God... the Word became flesh, and dwelt among us, and we beheld His glory, glory as of the only begotten from the Father, full of grace and truth") as indicating “Christ, God’s Word” and he thus substituted “Christ” for “the commandment.” He told us that God’s way of salvation was not difficult and complicated. We do not have to go to heaven to find Christ, or into the world of the dead. Christ is near to us. The Gospel of Christ, the Word of faith, is available and accessible. The sinner need not perform difficult works in order to be saved. All he has to do is trust Christ. The very Word on the lips of the religious Jews was the Word of faith. The very Law that they read and recited pointed to Christ.

And so the "righteousness based on faith" speaks out and says that the righteousness God provides is not unattainable. It is not in heaven or in the abyss (Ro 10:7), locations that even the most sincere and zealous man's fleshly efforts or works could never reach.

God's righteousness is present in the message, the word of (the) faith, which in the context of Romans is the Gospel which Paul has been proclaiming for the first 8 chapters. This word is effected (or realized) by exercise of personal faith, by confession with one's mouth and belief in one's heart (Ro 10:9-10). This is the truth that Paul is pressing home, the truth that this Gospel is not difficult and is not unattainable. It is only difficult or unattainable if you try to attain in your own strength and own effort, as if you could merit it. This simple truth deflates our pride and this dynamic is what must transpire if we would come and as little children receive the word implanted which is able to save our soul.

The context shows that the purpose of the apostle is to contrast the legal/works method and the Gospel method of salvation and thus to show that the one is impracticable, the other easy. By works of the law no one can be justified, whereas whoever simply calls on the name of the Lord will be saved.

Ascending and descending are acts men do and Paul is saying that you don't need to "work" or perform acts to obtain the righteousness of Christ. It's as simple as receive and believe.

Descend (2597) (katabaino from katá = down + baíno = go or come) means to come or go down and so to descend from a higher to a lower place.

Abyss (12) (abussos, from a = an intensifier + buthos =deep) means first an extremely deep or bottomless and speaks of the underworld which in Greek was the place of dead (although it is not the lake of fire). 

Paul substitutes "the deep" (abyss) for "the sea" in the Deuteronomy passage

"Nor is it beyond the sea, that you should say, 'Who will cross the sea for us to get it for us and make us hear it, that we may observe it?'