Heb 8:5-12, Heb 10:12-18, Acts 5:42, Acts 28:30-31
In the Old Testament, God spoke of God's new covenant, that is, that God's Word would never depart from our mouths. (Isa 59:21)
Isa 59:21 As for me, this is my covenant with them, saith the Lord; My spirit that is upon thee, and my words which I have put in thy mouth, shall not depart out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed's seed, saith the Lord, from henceforth and for ever.
The first covenant that God established in the Old Testament, the law, is the model of the new covenant. God's new covenant is the written word of God in our hearts. (Hebrews 8:5-12)
Hebrews 8:5 Who serve unto the example and shadow of heavenly things, as Moses was admonished of God when he was about to make the tabernacle: for, See, saith he, that thou make all things according to the pattern shewed to thee in the mount. 6 But now hath he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established upon better promises. 7 For if that first covenant had been faultless, then should no place have been sought for the second. 8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah: 9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord. 10 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: 11 And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest. 12 For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.
God's new covenant is that Christ died for our sins so that we may be perfected forever. (Hebrews 10:12-18)
Hebrews 10:12 But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God; 13 From henceforth expecting till his enemies be made his footstool. 14 For by one offering he hath perfected for ever them that are sanctified. 15 Whereof the Holy Ghost also is a witness to us: for after that he had said before, 16 This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them; 17 And their sins and iniquities will I remember no more. 18 Now where remission of these is, there is no more offering for sin.
The early church daily proclaimed that Jesus, the new covenant of God, is the Christ. (Acts 5:42)
Acts 5:42 And daily in the temple, and in every house, they ceased not to teach and preach Jesus Christ.
Even in prison, Paul testified of Christ Jesus, the new covenant prophesied in the Old Testament. (Acts 28:30)
Acts 28:30 And Paul dwelt two whole years in his own hired house, and received all that came in unto him,