John MacArthur writes in his notes on Jeremiah 4:4 NKJV the following concerning circumcision:
This surgery (Gen. 17:10-14) was to cut away flesh that could hold disease in its folds and could pass the disease on to wives. It was important for the preservation of God’s people physically. But it was also a symbol of the need for the heart to be cleansed from sin’s deadly disease. The really essential surgery needed to happen on the inside, where God calls for taking away fleshly things that keep the heart from being spiritually devoted to Him and from true faith in Him and His will. Jeremiah later expanded on this theme (31:31-34; cf. Deut. 10:16; 30:6; Roman. 2:29). God selected the reproductive organ as the location of the symbol for man’s need of cleansing for sin, because it is the instrument most indicative of his depravity, since by it he reproduces generations of sinners.*
Circumcise yourselves to the LORD, and take away the foreskins of your heart, ye men of Judah and inhabitants of Jerusalem: lest my fury come forth like fire, and burn that none can quench it, because of the evil of your doings. Jeremiah 4:4 KJV
*The MacArthur Study Bible notes, NKJV – copyright 1997 by Word Publishing, a division of Thomas Nelson, Inc. Used by permission. All rights reserved.