"The heart is deceitful above all things, and desperately wicked: who can know it?" - Jeremiah 17:9 KJV

That is why we must circumcise our heart, which is equated to repentance and water baptism in Jesus name.

"In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead. And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;" - Colossians 2:11-13 KJV

AND we must have God's law written upon our heart, which is equated to receiving the Holy Ghost. The day of Pentecost celebrated the giving of the Law (on stones) on Mount Sinai; it was also on the day of Pentecost that God's law was written on hearts instead of on stone when the Holy Ghost was poured out.

"This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh." - Galatians 5:16 KJV

"But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more." - Jeremiah 31:33-34 KJV

Peter proclaimed the New Covenant to the people on the Day of Pentecost.

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." - Acts 2:38-39 KJV