These things were clone in Bethany beyond the Jordan, where John was baptizing. £ The fact that John the Baptist, in the previous verses, recognizes the Messiah, and that in verses 31-33 he declares that knowledge to have followed the baptism and the sign then given to him, makes it obvious that the baptism and the forty days of the temptation are now in the past.
Every day is clearly marked from the day on which the deputation from the Sanhedrin approached him, till we find Jesus at Cana, on his way to Jerusalem. Consequently, the baptism of Christ, which was the occasion of the higher knowledge that John acquired concerning him, as well as the temptation, had been consummated.
Of this last it would seem highly probable John had received, in subsequent conversation with the Lord, a full report. The Lord had passed through the fiery ordeal. He had accepted the position of the Servant of the Lord, who, in the way of privation, suffering, fierce antagonism from world, flesh, and devil, would win the crown of victory and prove himself to be the Life and Light of the world.
This chronological hint appears to me to explain the sudden and surprising utterance of the next verse.