Mt 3:3, Isa 40:3, Mal 3:1, Mt 3:11, Jn 1:33-34, Mt 3:16, Isa 11:2, Mt 3:15, Jn 1:29, Mt 3:17, Ps 2:7
The Old Testament prophesies that there will be someone who will prepare the way for Christ. That person is John the Baptist. (Mt 3:3, Isa 40:3, Mal 3:1)
Mt 3:3 For this is he that was spoken of by the prophet Esaias, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
Isa 40:3 The voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God.
Mal 3:1 Behold, I will send my messenger, and he shall prepare the way before me: and the Lord, whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, whom ye delight in: behold, he shall come, saith the Lord of hosts.
John the Baptist prophesied that Christ would baptize with the Holy Spirit. (Mt 3:11)
Mt 3:11 I indeed baptize you with water unto repentance. but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
Also, John the Baptist testified that the one to whom the Holy Spirit came when he baptized was Christ. It was foretold in the Old Testament that the Holy Spirit would come upon Christ. (Jn 1:33-34, Mt 3:16, Isa 11:2, Is 42:1)
Jn 1:33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost. 34 And I saw, and bare record that this is the Son of God.
Mt 3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
Isa 11:2 And the spirit of the Lord shall rest upon him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and of the fear of the Lord;
Is 42:1 Behold my servant, whom I uphold; mine elect, in whom my soul delighteth; I have put my spirit upon him: he shall bring forth judgment to the Gentiles.
When John the Baptist baptized Christ, all the sins of the world were passed on to Him. (Mt 3:15, Jn 1:29)
Mt 3:15 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him.
Jn 1:29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
God confirmed by the word that Jesus was the Son of God who would accomplish the work of Christ. (Mt 3:17, Ps 2:7)
Mt 3:17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
Ps 2:7 I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee.