Heb 8:10-12, Acts 5:31-32
In the Old Testament, God spoke of giving the Holy Spirit of God into the hearts of the remnant of Israel to make them His people. (Ezek 11:17-20)
Ezek 11:17 Therefore say, Thus saith the Lord God; I will even gather you from the people, and assemble you out of the countries where ye have been scattered, and I will give you the land of Israel. 18 And they shall come thither, and they shall take away all the detestable things thereof and all the abominations thereof from thence. 19 And I will give them one heart, and I will put a new spirit within you; and I will take the stony heart out of their flesh, and will give them an heart of flesh: 20 That they may walk in my statutes, and keep mine ordinances, and do them: and they shall be my people, and I will be their God.
The writer of Hebrews quoted from the Old Testament and said that God had put the word of God in the hearts of the people of Israel so that they might come to know God. (Hebrews 8:10-12)
Hebrews 8:10 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.
As promised in the Old Testament, God poured out the Holy Spirit on those who believed in Jesus as the Christ. (Acts 5:31-32)
Acts 5:31 Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins. 32 And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him.