The Torah says that "ASHER YA'ASEH OTAM HA'ADAM VA'CHAI BAHEM" ("THE MAN THAT DOES THEM [THE COMMANDMENTS/WORKS OF THE LAW] SHALL LIVE BY THEM", LEV 18:5")  This is the way of legal justification.  See Romans 10:5.
Beginning with the commandments to procreate and be circumcised there are some 613 commandments (Thou shalt's and Thou shalt not's) which Leviticus 18:5 seems to suggest the observing of which will gain life, that indeed there are 613 stairsteps to heaven, that "if a man will do them, he will live by them." The non-Jew who wanted to become a proselyte was required to be circumcised and bear this entire yoke of the Law with faithfulness. Rav Sha'ul was falsely accused of teaching Jewish people that they could not circumcise their sons nor observe the customs of their religion (Acts 21:21); however, such was not the case at all. He was not speaking of the Jewish manner of life or the Sinai Covenant per se, but something else. Namely, there was another scripture that threw a gloomy shadow over legal justification as the way of salvation.

"ARUR ASHER LO YAKIM ES DIVREI HATORAH HAZOT LA'ASOT OTAM" ("CURSED IS EVERYONE WHO CONFIRMETH NOT IN ALL THINGS WHICH ARE WRITTEN IN THIS TORAH TO DO THEM" Deut.27:26) This is the curse of the Law, since the Torah spells it out. However, it is also the judicial curse of G-d under Whose wrath all of sinful humanity ("the heart of man is desperately wicked, who can know it?"--Jeremiah 17:9) already stands fallen and condemned. To be cursed is to be under the divine condemnation, as on the night of the first Passover, when the divine wrath threatened all the firstborn of the Land of Egypt and their accursed idols (Exodus 12:12). But Hashem showed the way of escape: "When I see the blood [of the perfect Lamb of G-d], I will pass over you (Exodus 12:13)," meaning you will be spared from the death curse and redeemed for salvation.

Here the Lamb of G-d is all-important, because Moshiach was led "KA'SEH LA'TEVACH" (LIKE A LAMB TO THE SLAUGHTER" Isaiah 53:7), for the Torah says "KILELAT ELOKIM TALUI AL ETZ" ("ACCURSED OF G-D IS THE ONE HANGED ON THE TREE" Deut.21:23). MOSHIACH HAS REDEEMED US FROM THE CURSE OF THE TORAH, BECOMING A CURSE FOR US, as it says, "WE DID ESTEEM HIM STRICKEN, SMITTEN OF G-D AND AFFLICTED" (Isaiah 53:4), but MOSHIACH TZIDKEINU ("Messiah our righteousness," Jer 23:5-6), HASHEM'S RIGHTEOUS SERVANT, SHALL JUSTIFY (Genesis 15:6; Habakkuk 2:4) MANY FOR HE WILL BEAR THEIR INIQUITIES (Isaiah 53:11).

So we see the importance of Moshiach in salvation.  Now there were many Pharisees who became convinced that Isaiah 53 was speaking about Moshiach and that the Good News must be preached.  However, the Besuras HaGeulah that these Messianic Pharisees preached was that unless you—-even if you were a Gentile proselyte—-unless you not only believed in Yeshua HaMoshiach but also kept all 613 mitzvot, you could not be saved (Acts 15:1; Ga 5:3).  Thus they endeavored to pour new wine into old wineskins, even to walk in the newness of life by the oldness of the letter. 

But there is a problem with this kind of Messianic Judaism which was not the Messianic Judiasm of Rav Sha'ul or Moshiach's Shliach Yaakov in Yerushalayim.  Their Shul in Yerushalayim was Shomer Shabbos and kosher but their mode of justification and sanctification was not according to the oldness of the letter.  These men spoke in tongues!  See Acts 1:14 and 2:4; and 1Co 14:18).  They had been filled with the new wine of the Ruach Hakodesh.  And they had been armed in a new and living way to become victors over the believer's ongoing struggle with sin.

There is a problem with sin even after a believer turns to Moshiach.  The sinful nature desires what is contrary to the Ruach Hakodesh and the Ruach Hakodesh desires what is contrary to the sinful nature Ga 5:17.  And when the Torah prohibits something, the sinful nature wants it more and more.  For example, what preacher has not preached the 10^th Commandment “Thou shalt not covet!” and then IMMEDIATELY found himself insanely jealous of another preacher?  The more the 10^th Commandment tells him this is prohibited, the more jealous and covetous the preacher becomes.  So chapter 7 of Romans is to show that the Derech Hakodesh is not the road taken by the Messianic Pharisee legalist.  This law, this commandment not to covet, even ESPECIALLY known in the oldness of the letter, will not win out over sin in the believer's struggle for sanctification.

Here I must stop to make an aside.  There are anti-Jewish preachers today who label everything "Judaizing" if it speaks of the Jewishness of the   

Gospel.  Even many scholars misread Ac 21:20-21 and think the text is saying "Many [Judaizers] were zealous for legal justification." The text says no such thing.  In spite of Ac 15:13 and following many scholars read Ga 2:12 and believe that a separatist Ya'akov sent the Jerusalem group to Antioch.  The text does not say that.  The fact is, there were brethren in Jerusalem that worshiped in Moshiach's Shul there and there were brethren outside of Jerusalem.  There was unity between them though they were culturally diverse.  We must all stand before Moshiach's Bet Din, and this humbles us and brings unity. However, if we know Moshiach's Torah, the Torah wherein Moshiach is the goal, the Torah wherein everything is about Moshiach (Lk 24:44), then we can approach with confidence and not shrink back at the Bias HaMoshiach.  When the Torah of Moshe Rabbeinu is set forth truthfully, it is the Derech Moshiach, the Torat HaCherut (the Law of Freedom).  He whom the Moshiach sets free is free indeed.  To say this is not reading the Torah with the intent of legalism or judaizing.  This is not adding conditions to salvation.  This is not telling Goyim that they have to do certain Jewish mitzvos to be saved. Neither Sha'ul nor Ya'akov offers the law in itself as a means to salvation (Jas. 2:10; Gal. 3:10). Both men speak of the "torah of freedom" (Jas. 1:25; 2:12; 1 Cor. 9:21) in a way that implies Moshiach (Jas. 2:1; Ro. 8:2), and Ya'akov, no less than Sha'ul, emphasizes the need for emunah (Jas. 1:3, 6; 5:15), for ahavah (agape) toward G-d (Jas. 1:1), and being reborn from above (Jas. 1:21).

Romans chapter 7 is Paul's reflection on the error of the Messianic Pharisee who tries to follow Yeshua HaMoshiach as a legalist.  613 commandments followed according to the oldness of the letter will indict sin, but because of the carnal nature, the 613 commandments will also make to spring to life that remnant of the old nature still present in the born again believer.

Let's explain. In Rav Sha'ul's tightly reasoned exposition of the Good News of Redemption he says something about being born again.  He says I have died to sin, and I have been raised to newness in Moshiach.  But in Chapter 7 he tells us that he still has a sinful nature. He has the desire to do what is right because he is a new creature in Moshiach; nevertheless, because of remnants of the old nature, mere legalistic religious activity on his part will not perfect him because regulations in and of themselves, good and holy and just though they may be, will not restrain sensual indulgence.

It is only when you become born again that you really begin to understand the wickedness of your own heart and the deeper demands of G-d holy Torah.  Before you were regenerated you thought the Torah was a thing you really had a handle on, and you prided yourself on that.  You actually thought you were a pretty decent person, especially since you observed your religion as well as you did.  But the Ruach Hakodesh when he entered your life brought all that presumption and pride to a close.  Now you see that you don't know the Torah; the Torah knows you, and that you are a sinner!  When the Messianic Pharisee says, "O wretched man that you are, knowing some of the wickedness in your heart, listen, just obey the Torah and you will have assurance of salvation!" Rav Sha'ul knew that the law was not his savior, Yeshua HaMoshiach is his savior (see Ro 7:24-25).

Now if you were attending Moshiach's Shul in Yerushalayim on Shabbos in 57 C.E. and you saw Rav Sha'ul and the Shliach Ya'akov davening and the Messianic Jews reading the Sefer Torah and worshipping, you might be hard pressed to tell who in the audience were Messianic Pharisaic legalists and who weren’t, but when you heard the leshonot chadashot coming from the mouths of Rav Sha'ul and Shliach Ya'akov you would know that these two men's sanctification was a matter of the work of the Ruach Hakodesh and not their walk in the oldness of the letter, even though the atmosphere of this Shul would be glatt kosher (1C 9:20) as far as Torah observance was concerned. 

Today there are many Messianic Pharisees and anti-Jewish anti-charismatic legalists from non-Jewish backgrounds who have joined the Messianic Jewish movement or become critics of it and they don't seem to understand that Romans chapter 8 is referring to groanings without words, groanings that the human mind cannot comprehend, groanings not in intelligible words but in leshonot chadashot, because the Ruach Hakodesh is the giver of the utterance (see OJB Ac 2:4; and 1Co 14:2; and Ro 8:26-27).  It is not in our own strength or in the oldness of the letter that we overcome the evil within the believer that Rav Sha'ul points to in Ro chapter 7.  Even in the midst of all the Yiddishkait of Messianic Judaism, it is the Ruach Hakodesh that brings us to Moshiach who gives us the victory.

If we point Jewish people to the truth of the Scriptures using Rabbinic writings as a bridge to the closed canon of inerrant Holy Scripture, this is following the methodology endorsed by the Word. For example, 

Sanhedrin 98b says the World was created only on Moshiach's account.

לא  not

אברי  created

עלמא  the world

אלא  except

למשה  for Moshiach. 

Co 1:16 (OJB) says all things were created FOR Moshiach. 

See the ORTHODOX JEWISH BIBLE.