23 Thus says the LORD: “Let not the wise man glory in his wisdom, Let not the mighty man glory in his might, Nor let the rich man glory in his riches; 24 But let him who glories glory in this, That he understands and knows Me, That I am the LORD, exercising lovingkindness, judgment, and righteousness in the earth. For in these I delight,” says the LORD. Jeremiah 9:23–24 (NKJV) 

23 Thus says the Lord: “Let the wise not boast in his wisdom, and let the strong not boast in his strength, and let the wealthy not boast in his riches. 24 Rather let the one who boasts boast in this: that he understands and knows that I am the Lord, the one who deals in mercy and judgment and righteousness upon the earth. For my will is in this,” says the Lord. Jeremiah 9:23–24 (LES2) 

This passage is paraphrased by Paul.

31 that, as it is written, “He who glories, let him glory in the LORD.” 1 Corinthians 1:31 (NKJV) 

Interestingly, Paul says the same thing in another verse in Galatians, where he connects it to circumcision.

14 But God forbid that I should boast except in the cross of our Lord Jesus Christ, by whom the world has been crucified to me, and I to the world. 15 For in Christ Jesus neither circumcision nor uncircumcision avails anything, but a new creation. Galatians 6:14–15 (NKJV) 

The Jews boasted in circumcision being the sign of their special relationship with God, but both Jeremiah and Paul negate this claim.

25 “Behold, the days are coming,” says the LORD, “that I will punish all who are circumcised with the uncircumcised—26 Egypt, Judah, Edom, the people of Ammon, Moab, and all who are in the farthest corners, who dwell in the wilderness. For all these nations are uncircumcised, and all the house of Israel are uncircumcised in the heart.” Jeremiah 9:25–26 (NKJV) 

25 “Look! The day is coming,” says the Lord, “and I will visit upon all the circumcised their foreskins, 26 upon Egypt and upon Idumea and upon Edom and upon the sons of Ammon and upon the sons of Moab and upon everyone who shaves the things along his face, those who dwell in the desert, because every nation is uncircumcised in flesh, and the whole house of Israel is uncircumcised in their heart.” Jeremiah 9:25–26 (LES2) 

The central point in both the MT and LXX is that disobedient and unbelieving Israel is no different from the Gentiles because God looks at the heart, not upon outward things like circumcision. This is why Paul wrote what he did in the quote from Galatians above. In Romans, Paul stated it even more clearly.

25 For circumcision is indeed profitable if you keep the law; but if you are a breaker of the law, your circumcision has become uncircumcision. 26 Therefore, if an uncircumcised man keeps the righteous requirements of the law, will not his uncircumcision be counted as circumcision? 27 And will not the physically uncircumcised, if he fulfills the law, judge you who, even with your written code and circumcision, are a transgressor of the law? 28 For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; 29 but he is a Jew who is one inwardly; and circumcision is that of the heart, in the Spirit, not in the letter; whose praise is not from men but from God. Romans 2:25–29 (NKJV) 

As we study the prophets, we see from where Paul derived his doctrine. In God’s eyes, circumcision is simply an outward sign that was meant to accompany an inward reality.

9 Does this blessedness then come upon the circumcised only, or upon the uncircumcised also? For we say that faith was accounted to Abraham for righteousness. 10 How then was it accounted? While he was circumcised, or uncircumcised? Not while circumcised, but while uncircumcised. 11 And he received the sign of circumcision, a seal of the righteousness of the faith which he had while still uncircumcised, that he might be the father of all those who believe, though they are uncircumcised, that righteousness might be imputed to them also, 12 and the father of circumcision to those who not only are of the circumcision, but who also walk in the steps of the faith which our father Abraham had while still uncircumcised. Romans 4:9–12 (NKJV) 

The inward reality is what really matters to God, whether or not accompanied by outward circumcision. Confessing that Jesus is Lord reveals whether there is a circumcision of the heart that produces a new creation – the born-again person. This applies to all people, both Jew and Gentile, which is the glorious reality of the New Covenant.