Galatians 2:15-16

"We who are Jews by birth and not 'Gentile sinners' know that a man is not justified by observing the law, but by faith in Jesus Christ. So we, too, have put faith in Christ Jesus that we may be justified by faith in Christ and not by observing the law, because by observing the law no one will be justified."

We who are Jews by birth
Philippians 3:4b-5 If anyone else thinks he has reasons to put confidence in the flesh, I have more: circumcised on the eighth day, of the people of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; in regard to the law a Pharisee; ...

and not 'Gentile sinners'
1 Samuel 15:18 "And he sent you on a mission, saying, 'Go and completely destroy those wicked people, the Amalekites; make war on them until you have wiped them out.'"
Luke 24:7 "'The Son of Man must be delivered into the hands of sinful men, be crucified and on the third day be raised again.'"

know that a man is not justified by observing the law
Romans 3:28 For we maintain that a man is justified by faith apart from observing the law.
Romans 7:12 So then, the commandment is holy, righteous and good.

but by faith in Jesus Christ
Romans 9:30 What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; ...
Romans 3:20 Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin.
Romans 3:23-24 ... for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus.
Philippians 3:9 ... and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith in Christ--the righteousness that comes from God and is by faith.

because by observing the law no one will be justified
Galatians 4:25 Now Hagar stands for Mt. Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children.

NOTES:
Read Romans 8

Scriptures taken from the Holy Bible, New International Version® NIV®, Copyright© 1973, 1978, 1984 by International Bible Society. Used by permission. All rights reserved worldwide.