Mal 4:5, Mk 1:2-4, Mk 9:11-13, Lk 1:13-17, Lk 1:76, Lk 7:24-27, Mt 11:1-5,10-14, Mt 17:10-13, Acts 19:4

In the Old Testament, God said that an angel of God would prepare the way for Christ. (Mal 3:1, Mal 4:5)

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.

Mal 4:5 Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord:

An angel appeared to Zacharias and told him that the child his wife would bear would prepare the way for Christ in the spirit of Elijah. (Luke 1:13-17,76)

Luke 1:13 But the angel said unto him, Fear not, Zacharias: for thy prayer is heard; and thy wife Elisabeth shall bear thee a son, and thou shalt call his name John. 14 And thou shalt have joy and gladness; and many shall rejoice at his birth. 15 For he shall be great in the sight of the Lord, and shall drink neither wine nor strong drink; and he shall be filled with the Holy Ghost, even from his mother's womb. 16 And many of the children of Israel shall he turn to the Lord their God. 17 And he shall go before him in the spirit and power of Elias, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord. 76 And thou, child, shalt be called the prophet of the Highest: for thou shalt go before the face of the Lord to prepare his ways;

As prophesied in the Old Testament, an angel of God appeared to prepare the way for Christ. That messenger is John the Baptist. (Mark 1:2-4, Mark 9:11-13, Luke 7:24-27, Matthew 17:10-13, Matthew 11:10-14)

Mark 1:2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee. 3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight. 4 John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins.

Mark 9:11 And they asked him, saying, Why say the scribes that Elias must first come? 12 And he answered and told them, Elias verily cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought. 13 But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him.

Luke 7:24 And they asked him, saying, Why say the scribes that Elias must first come?

12 And he answered and told them, Elias verily cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought.

13 But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him.

Matthew 17:10 And his disciples asked him, saying, Why then say the scribes that Elias must first come? 11 And Jesus answered and said unto them, Elias truly shall first come, and restore all things. 12 But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them. 13 Then the disciples understood that he spake unto them of John the Baptist.

Matthew 11:10 For this is he, of whom it is written, Behold, I send my messenger before thy face, which shall prepare thy way before thee. 11 Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he. 12 And from the days of John the Baptist until now the kingdom of heaven suffereth violence, and the violent take it by force. 13 For all the prophets and the law prophesied until John. 14 And if ye will receive it, this is Elias, which was for to come.

John the Baptist had his disciples ask Jesus if he was the Christ foretold that Jesus would come. Then Jesus said that the prophecy of Christ in the Old Testament had been fulfilled to himself, and he revealed that he was the Christ. (Mt 11:1-5)

Mt 11:1 And it came to pass, when Jesus had made an end of commanding his twelve disciples, he departed thence to teach and to preach in their cities. 2 Now when John had heard in the prison the works of Christ, he sent two of his disciples, 3 And said unto him, Art thou he that should come, or do we look for another? 4 Jesus answered and said unto them, Go and shew John again those things which ye do hear and see: 5 The blind receive their sight, and the lame walk, the lepers are cleansed, and the deaf hear, the dead are raised up, and the poor have the gospel preached to them.

Paul testified that the Christ John the Baptist was talking about was Jesus. (Acts 19:4)

Acts 19:4 Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.