1 Corinthians 9:20

And, to the Jews, I became as a Jew, that I might gain Jews; to those under law, as under law (not being myself under law), that I might gain those under law;

Acts 16:3

this one Paul wished to go forth with him; and, taking him, he circumcised him because of the Jews who were in those places; for they all knew that his father was a Greek.

Acts 21:20-26

And they, hearing it, were glorifying God, and said to him, "You observe, brother, how many thousands there are among the Jews, of those who have believed; and they are all zealous for the law.

Acts 17:2-3

and, according to Paul's custom, he went in to them; and for three sabbaths reasoned with them from the Scriptures,

Acts 18:18

And Paul, abiding after this yet many days with the brethren, having taken leave of them, was sailing away to Syria, and with him Priscilla and Aquila; having shaven his head in Cenchrea; for he had a vow.

Romans 3:19

Now we know that whatsoever the law says, it speaks to those under the law; that every mouth may be stopped, and that the whole world may be under the sentence of God;

Romans 6:14-15

For sin shall not have dominion over you; for ye are not under law, but under grace.

Romans 11:14

if by any means I may excite to rivalry my flesh, and save some of them.

Galatians 4:5

that He might redeem those under law, that, we might receive the adoption of sons.

Galatians 4:21

Tell me, ye who are wishing to be under law, do ye not hear the law?

Galatians 5:18

But, if ye are being led by the Spirit, ye are not under law.

Treasury of Scripture Knowledge

Summary

And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;

Bible References

Unto

Acts 16:3
this one Paul wished to go forth with him; and, taking him, he circumcised him because of the Jews who were in those places; for they all knew that his father was a Greek.
Acts 17:2
and, according to Paul's custom, he went in to them; and for three sabbaths reasoned with them from the Scriptures,
Acts 18:18
And Paul, abiding after this yet many days with the brethren, having taken leave of them, was sailing away to Syria, and with him Priscilla and Aquila; having shaven his head in Cenchrea; for he had a vow.
Acts 21:20
And they, hearing it, were glorifying God, and said to him, "You observe, brother, how many thousands there are among the Jews, of those who have believed; and they are all zealous for the law.

Are under

Romans 3:19
Now we know that whatsoever the law says, it speaks to those under the law; that every mouth may be stopped, and that the whole world may be under the sentence of God;
Romans 6:14
For sin shall not have dominion over you; for ye are not under law, but under grace.
Galatians 4:5
that He might redeem those under law, that, we might receive the adoption of sons.
Galatians 5:18
But, if ye are being led by the Spirit, ye are not under law.

General references

Luke 11:37
Now, as He spake, a Pharisee asks Him to dine with him; and, entering, He reclined at table.
Romans 15:2
Let each one of us please his neighbor in that which is good for his edification;
1 Corinthians 10:33
even as I also, in all things, please all men: not seeking my own profit, but that of the many, that they may be saved.
Galatians 2:3
But not even Titus, who was with me, being a Greek, was compelled to be circumcised;