1. The Holy Spirit testifies of Christ. (Heb 10:15-18)
    Jer 31:33-34, Acts 5:30-32, Jn 14:26, Jn 15:26, Jn 16:13

In the Old Testament it was foretold that God’s law would be written in our hearts. (Jer 31:33-34)

Jer 31:33 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.
34 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.

The Holy Spirit has come upon those who believe that Jesus is the Christ. (Acts 5:30-32)

Acts 5:30 The God of our fathers raised up Jesus, whom ye slew and hanged on a tree.
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.

The Holy Spirit makes us realize that Jesus is the Christ. (Jn 14:26, Jn 15:26, Jn 16:13)

Jn 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.

Jn 15:26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:

Jn 16:13 Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.

The Holy Spirit has come into our hearts to make us understand in our hearts the Word of God, that Jesus is the Christ. (Heb 10:15-18)

Heb 10:15 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.