The voice of one crying in the wilderness:
"Prepare the way of the LORD;
Make straight in the desert
A highway for our God.
Every valley shall be exalted
And every mountain and hill brought low;
The crooked places shall be made straight
And the rough places smooth;
The glory of the LORD shall be revealed,
And all flesh shall see it together;
For the mouth of the LORD has spoken."
This was a prophesy about the coming Christ. Someone would come before Him to get everybody ready. Now what do we know about Old Testament prophesies?
They have all come true, or will all come true. This prophesy about John the Baptist did come true, and the Gospel of Mark even puts very clearly. It gives the first 2 of those verses above and then talks about John came baptizing for repentance of sins and gives a little information about how he was.
How do we know that Jesus is the Messiah, the Christ, the Son of God? This is the way we do: every prophesy in the Old Testament about Him was fulfilled in some way in Jesus. This is one of them. There were signs, like John the Baptist. The other example I can think of off the top of my head is how the soldiers divided Jesus' clothes when He was being crucified, which was foretold in one of the prophets of the Old Testament.
If your Bible gives the cross references, I would encourage you to go through the four gospels and read every single verse they mention in the Old Testament. It strengthens our faith to have bits of proof of our God. Now, some people don't need that proof, they go off of how they know in their heart that God is real, but for those of us who need some sort of proof, the fact that this Jesus fulfilled every prophesy written about the Messiah, especially those written many years before His birth, can be extremely reassuring (or assuring).
In His Service,