Question: "Circumcise therefore the foreskin of your heart, and be no longer stubborn." Deut. 10:16 --> If God commands the Israelites to circumcise their own hearts in this verse and in Jer. 4:4, how can we reconcile these two verses with monergistic principles?
Answer: Good question.
Let me first answer with a question. God commands you love love Him with all your heart mind and strength and your neighbor as yourself. Can you do so apart from grace?
When Adam fell in the garden God did not change his holy standard for him or us. Although corrupted, God still commands all men everywhere to be holy, and he punishes men for not obeying his law perfectly. This demonstrates that our inability does not alleviate our responsibility - God commands holiness even though we are impotent to be holy. This is the whole point of the gospel where He extends grace in Jesus Christ - TO GRANT WHAT HE COMMANDS. And we see this very principle later in (the book you cite) Deuteronomy 30:6 where God declares: “Moreover the Lord your God will circumcise your heart and the heart of your descendants, to love the Lord your God with all your heart and with all your soul, so that you may live."
He grants in Deuteronomy 30:6 what he commanded in Deuteronomy 10:16.
Remember Romans 3:19, 20 teach us the purpose of the commands in the Bible >>> "through the law comes knowledge of sin"... In other words, the purpose of commands is not to show our ability but our inability. Carried over to Deut 10:16 this means the command to circumcise our own hearts is God's holy requirement for us, but can only be fulfilled by grace.
God commands us to circumcise our hearts because this is his holy requirement for us. And then in his mercy He grants the very thing he commands (Deut 30:6). Jesus came to do for us what we were unable to do for ourselves.
As for Jeremiah we see the same principle, The entire book of Jeremiah is about a prophet sent to people who, God tells him, will not listen. "So you shall speak all these words to them, but they will not listen to you. You shall call to them, but they will not answer you." (Jeremiah 7:27) So the command to circumcise their own hearts is going to people who God declares will reject his message.
Like Deuteronomy 30:6, there are also similar promises for regenerating grace in Jeremiah.
Jeremiah 24:7 I will give them a heart to know that I am the Lord, and they shall be my people and I will be their God, for they shall return to me with their whole heart.
For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people.
I will make with them an everlasting covenant, that I will not turn away from doing good to them. And I will put the fear of me in their hearts, that they may not turn from me.
Hope this helps