Heart defined as the seat of moral life and character
[Heart is:] The spring of all our actions and purposes; the seat of moral life and character; the moral affections and character itself; the individual disposition and character; as, a good, tender, loving, bad, hard, or selfish heart. Webster’s Revised Unabridged Dictionary
Heart is the first place God works His creation of new creatures
By God’s grace, we believe
We believe because, by His grace, God gave us His gift of faith (Eph 2:4-5; Eph 2:8). It is with our heart that we believe.
But what does it say? “The word is near you; it is in your mouth and in your heart,” that is, the message concerning faith that we proclaim: If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved. As Scripture says, “Anyone who believes in him will never be put to shame.” Ro 10:8-11 KJV
The moment we believe, God creates us in Christ Jesus
He has made us what we are, creating us in Christ Jesus to do good works, in which God long ago planned for us to live. Eph 2:4-10 AAT
For the love of Christ constraineth us; because we thus judge, that if one died for all, then were all dead: And that he died for all, that they which live should not henceforth live unto themselves, but unto him which died for them, and rose again.
Wherefore henceforth know we no man after the flesh: yea, though we have known Christ after the flesh, yet now henceforth know we him no more. Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. And all things are of God, who hath reconciled us to himself by Jesus Christ, and hath given to us the ministry of reconciliation; To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation. 2 Cor 5:14-19 KJV
God has created us in Christ Jesus and cleansed by His blood.
Being created in Christ Jesus is an instantaneous salvation process called regeneration. The moment we confessed Jesus is the Son of God, God dwelt within us and we in Him (1 Jn 4:15), creating us in Christ Jesus. “Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost” (Titus 3:5 KJV). N. T. Wright puts washing of regeneration “washing of the new birth” (The Kingdom New Testament). Regeneration is the again genesis of the New Covenant. “Regeneration” comes from the Greek word “paliggenesia” from the root words “palin/again” and “genesis.” Thus God created us in Christ Jesus as New beings fit for the kingdom of God. The only other time “regeneration” is used in the Bible, Jesus is referring to the again genesis of the righteous and heaven and earth (Mt 19:28; Rev 21-22). Thus, the outpouring of the Holy Spirit was the beginning of a new order of mankind, alive unto God. Thus, God makes us new beings fit for the kingdom of God. Also see: We are born of the Spirit and we are spirit; We walk by faith; Jerusalem Spiritual Jerusalem
Through regeneration, God gives us a new heart.
Forasmuch as ye are manifestly declared to be the epistle of Christ ministered by us, written not with ink, but with the Spirit of the living God; not in tables of stone, but in fleshy tables of the heart. 2 Co 3:3
Our new heart is a blessing of the new covenant.
For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people. Heb 8:10 KJV. CF: Heb 10:16
A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you a new heart of flesh. Ezekiel 36:26 KJV CF: Jer 24:7; Jer 31:33
In Christ Jesus, we Jews and Gentiles are made one new man, a new order of mankind.
But now in Christ Jesus you who once were so far away have been brought near through the death of Christ. For He is our peace—He who has made Jews and Gentiles one, and in His own human nature has broken down the hostile dividing wall, by setting aside the Law with its commandments, expressed, as they were, in definite decrees. His design was to unite the two sections of humanity in Himself so as to form one new man. Eph 2:13-16 WNT
Let us receive what Paul prayed for the saints in Ephesus and for us.
Therefore I entreat you not to lose heart in the midst of my sufferings on your behalf, for they bring you honor. For this reason, on bended knee I beseech the Father, from whom the whole family in Heaven and on earth derives its name, to grant you—in accordance with the wealth of His glorious perfections—to be strengthened by His Spirit with power penetrating to your inmost being. I pray that Christ may make His home in your hearts through your faith; so that having your roots deep and your foundations strong, in love, you may become mighty to grasp the idea, as it is grasped by all God’s people, of the breadth and length, the height and depth—yes, to attain to a knowledge of the knowledge-surpassing love of Christ, so that you may be made complete in accordance with God’s own standard of completeness. Now to Him who, in exercise of His power that is at work within us, is able to do infinitely beyond all our highest prayers or thoughts—to Him be the glory in the Church and in Christ Jesus to all generations, world without end! Amen. Eph 3:13-21 WNT