Three times in one sentence, apparently.
Any of you parents? Any of you need to verbally pound the exact. same. message. into the rock-like brain of your child? Then you can sympathize with Paul here. Paul says three times that a man is not justified by the works of the law. I can't give you multiple verse citations, because they all come from verse 16!
What are works of the law? The law was that system of commandments that God gave Israel, beginning with the Ten Commandments (though the very-very first law God gave Israel was to Abram about circumcision, in Genesis 17). You remember the Ten Commandments? 1. Have no other gods before me (Ex. 20:3). 2. Make no images of worship (20:4-6). 3. Do not take the Lord's name in vain (20:7). 4. Keep the Sabbath (20:8-11). 5. Honor your parents (20:12). 6. Do not murder (20:13). 7. Do not commit adultery (20:14). 8. Do not steal (20:15). 9. Do not bear false witness (20:16). 10. Do not covet (20:17).
These are the works of the law, plus all the other commandments found in Exodus and Leviticus, later repeated in Deuteronomy.
I have a book in my library, written by a Roman Catholic apologist, who claims that "the law" meant "bad motives." This is obviously false. All Jews knew what the law was. The Jews built their lives around the law, or at least their culture as a whole tried to. Paul was a rabbi and a trained theologian. He certainly knew what the law was.
What does it mean to be justified? It does not mean to be more "holy in your heart." If you read Paul in the book of Romans 4:5-8, you can see that justification is something God accounts to you (5b). You are credited with just-ness. David said it was something God imputes to you (6, 8), like a jury declaring you innocent of all charges. David equates justification with being forgiven, which is something God does for you, not in you (7). It is when God covers your sins, so that He no longer "sees" them (7).
Justification is the Gospel. New birth is not the main feature of the Gospel. Justification is the root of all blessing. And it is never by works.
It is not by works at the start of your Christian life. It is not by works in the middle of your Christian life. It is not by works at the end of your Christian life here on earth. Your works show other people that you have faith, but it is the faith that justified you.
This is Christianity -- that you are not justified by works, Seventh-Day Adventists reject this, which is why they are not a Christian group. People who say that you must lead a godly life in order to "keep yourself saved" show they are ignorant of the Gospel. Christians who say that the ordinances of the church, baptism and the Lord's supper, somehow help save you, show themselves destructively and dangerously confused about the Gospel (this latter error is called "legalism" or "Christian Phariseeism").
How many times does the Spirit of God need to repeat to you that you are not justified by works, until you surrender? Until you finally obey the Lord and give up trying to attain, or maintain, justification by works?