Mark 1:5

Multitudes of Jews responded enthusiastically to John’s ministry. Large crowds from southern Palestine and Jerusalem went to the Jordan River in response to his call to prepare for Messiah’s appearance. Mark’s use of "all" was hyperbolic. Every individual did not come out to John, but very many did. Those who did confessed their sins by submitting to baptism. By allowing the forerunner of Messiah to baptize them, the Jews who submitted to his baptism were pledging to receive Messiah when He came.