In Matthew 16:15, Jesus’ question is not about his identity in itself, but whether his disciples acknowledge him as he presents himself—as the Son of Man. He wants not only faith in the heart but also confession with the mouth (see Romans 10:10), so the disciples—who already know who he is—must still give, in contrast to the people (but ... you,
as opposed to the non-disciples mentioned in Matthew 16:13), a true, audible confession concerning him.
In Matthew 16:15 Jesus asks his disciples: But who do you say that I am?
The contrast with the previous question in Matthew 16:13 does not primarily pertain to the people who do not yet know versus the disciples who are the first to know, but the contrast pertains to the distinction between those who dismiss Jesus’ claim and reduce him to the role of a mere forerunner, and the disciples who have come to recognize who he truly is. The question, therefore, is not about his identity in itself, but whether his disciples acknowledge him as he presents himself—as the Son of Man. 1
Jesus asks them because he desires not only faith in the heart but also the confession with the mouth (see Romans 10:10). Therefore, the disciples—who already knew who he was—were nevertheless called upon here to provide, in contrast to the people (but... you,
as opposed to the non-disciples mentioned in Matthew 16:13), a true, audible confession concerning him.2
15 He said to them, “But who do you say that I am?”