 They were all aware of the fact that John the Baptist himself had given testimony that this is the coming one. This is the Lamb of God who takes away the sin of the world. They were aware that at that time when John baptized Jesus, the Spirit of God came down, descending upon him in divine affirmation. The Father's voice came out of heaven. This is my beloved Son in whom I am well pleased. There were all kinds of sort of events all the way through that indicated divine intervention from angels and from God himself and the Holy Spirit.