Jeremiah 9

V.1-3

These verses are from the mouth of The Lord.  He laments over 1. His people's suffering and 2. His people's sin.  The sins that He mentions are 1. Lies, 2. Adultery, and 3. Not knowing Him.

V.4-8

These verses elaborate on Judah's falseness.  They weary themselves in committing iniquity (V.5).  These people are clearly very different than the people who God cut a covenant with.  They refuse to know Him (V.6).

V.9

God has no choice but to end the covenant, as they clearly already have broken.

V.10

This weeping over the total destruction of Judah may be either God's or Jeremiah's, but it's most likely both.

V.12

God refers again to the supposed wisdom that the teachers of the Law in Judah claimed to have, rhetorically asking if they could explain why the desolation took place.

V.13-16

God gives the answer to His rhetorical question: because the people have forsaken His Law.  As new covenant believers, we can rest in the fact that Jesus has fulfilled the Law that Judah had forsaken.

V.23-24

Thus says the Lord, "Let not a wise man boast of his wisdom, and let not the mighty man boast of his might, let not a rich man boast of his riches; but let him who boasts boast of this, that he understands and knows Me, that I am the Lord who exercises lovingkindness, justice and righteousness on earth; for I delight in these things," declares the Lord. (Jeremiah 9:23, 24 NASB)

Knowing God is greater than worldly wisdom, physical stature/health, and material wealth.  God exercises loving kindness, justice, and righteousness equally.  He is both kind and severe.  He is perfect in justice and in love.  That explains His dealing with Judah and it explains the new covenant of the Gospel for modern day Christians.  These verses harmonize and explain the covenants of Scripture well.

V.25-26

There were many nations in the Middle East at that time that were practicing circumcision - the sign of God's covenant with Israel.  God says that Israel is "circumcised and yet uncircumcised" because they haven't had their hearts circumcised.  God isn't looking for outward religious customs.  He wants our true reverent worship.