The Gospel at Mass, from the beginning of the Gospel of Mark, speaks of the ministry and message of John the Baptist.
From Mark Chapter 1:
The beginning of the gospel of Jesus Christ the Son of God.
As it is written in Isaiah the prophet: Behold, I am sending my messenger ahead of you; he will prepare your way.
A voice of one crying out in the desert: “Prepare the way of the Lord, make straight his paths.”
John the Baptist appeared in the desert proclaiming a baptism of repentance for the forgiveness of sins.
People of the whole Judean countryside and all the inhabitants of Jerusalem were going out to him and were being baptized by him in the Jordan River as they acknowledged their sins.
John was clothed in camel’s hair, with a leather belt around his waist. He fed on locusts and wild honey. And this is what he proclaimed:
“One mightier than I is coming after me. I am not worthy to stoop and loosen the thongs of his sandals. I have baptized you with water; he will baptize you with the Holy Spirit.”
Deacon Greg has an excellent homily on this Gospel.
In regard to John the Baptist, he notes, "... he doesn’t necessarily appear when or how we expect him to. This preacher isn’t Joel Osteen in a silk suit and an air-conditioned arena. John the Baptist is rough and wild and frightening, yelling in the desert."