The Catechism in Questions & Answers

96. Who prepared the way for Jesus?

John the Baptist was the one who prepared the way for Jesus. This forerunner of Jesus promised by God (cf. Malachi 3: 1) preached repentance and proclaimed Jesus Christ as the Redeemer: “I indeed baptise you with water unto repentance, but He who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptise you with the Holy Spirit and fire” (Matthew 3: 11).
According to the biblical record, John the Baptist was the first to expressly refer to Jesus as the Son of God and also proclaim this to the people.

“There was a man sent from God, whose name was John. This man came for a witness, to bear witness of the Light, that all through him might believe. He was not that Light, but was sent to bear witness of that Light.”
John 1: 6-8
“And I have seen and testified that this is the Son of God.”
John 1: 34