He always told people that the Saviour of the World would be coming soon.
John baptised people in the River Jordan.
One Day, Jesus came to the River Jordan because he wanted John to baptise him. However, John tried to talk Jesus out of it.
John ended up doing it. After Jesus came out of the water, the heavens opened on top of the river. John saw the Holy spirit, and it came down as a dove to rest on Jesus.
A voice from heaven proclaimed: “This is my own dear son with whom I am pleased.” John then knew that Jesus was the person everyone was waiting for.