Possible meanings are 1) that the people did not know that Jesus was the Messiah or 2) that the people did not understand the significance of what they were doing.
# God foretold by the mouth of all the prophets
When the prophets spoke, it was as though God himself was speaking because he told them what to say. AT: "God foretold by telling all of the prophets what to speak"
# foretold
"told ahead of time" or "told before it happened"
# the mouth of all the prophets
Here the word "mouth" refers to the words that the prophets spoke and wrote down. AT: "the words of all the prophets" (See: [[rc://en/ta/man/translate/figs-metonymy]])