“But when Herod heard these words, he said, “John, whom I beheaded, has been raised.” The evidence in Scripture is that Mark 9:16 is the first time that Herod declared that Jesus was John the Baptist raised from the dead.
Herod himself had no personal knowledge about Jesus. Herod had never personally met Jesus until Jesus was arrested in Jerusalem and sent to Pontius Pilate, who then sent him to Herod (Luke 23:6-8). So Herod said what he did about Jesus—that he was John the Baptist who had been raised from the dead and that was why he could do miracles—only because that was what he heard people say and it convinced him (cp. Mark 6:14; Luke 9:7).
The Bible tells us that when Herod first heard about Jesus, he did not know what to think; he was “greatly perplexed” (Luke 9:7). Herod heard from some people that this Jesus was John the Baptist; from other people that Jesus was Elijah; and from other people that Jesus was one of the Old Testament prophets who had been raised from the dead (Luke 9:7-8; Mark 6:14-15 REV). Upon hearing the three possibilities, Herod did not know what to think and said, “Who, then, is this about whom I hear such things?” (Luke 9:9).
At some point, and for some unstated reason, Herod decided that Jesus was John the Baptist raised from the dead (Matt. 14:2; Mark 6:16). Herod parroted what he had heard but had no personal knowledge of, that Jesus was John raised from the dead and that that was why Jesus could do the miracles that he did (Luke 9:7; cp. Matt. 14:2).
[For more on Mark 6:14 and 6:16, see commentary on Mark 6:14.]