Why does Jesus introduce his statement regarding the greatness of John the Baptist with the words Truly, I say to you?
In short
Jesus says Truly, I say to you
to solemnly assure the crowds, and demand their belief, that no prophet (see Matthew 24:11, Matthew 24:24; John 7:52) has arisen who is greater than John the Baptist.
With the words Truly, I say to you,
Jesus provides a solemn assurance regarding John the Baptist that demands acceptance through faith. Among all those born of women—among all human beings—no prophet (see Matthew 24:11, Matthew 24:24; John 7:52) has arisen who is greater than John the Baptist. For the listening crowds, this affirmed their already pre-existing positive view of the great prophet John. Yet Jesus here again highlights this point emphatically: John is the greatest of all.1
11 Truly, I say to you, among those born of women there has arisen no one greater than John the Baptist. Yet the one who is least in the kingdom of heaven is greater than he.