Luke 9:21

And he strictly charged them, and commanded them
Though he highly approved of this their confession, and pronounced Peter blessed upon it; and signified that it was a discovery which flesh and blood could not make, but what was made to him his Father: yet he gave them a strict charge, and laid his commands on them,

to tell no man that thing;
that he was the Messiah, and the eternal Son of God, and the true God, as well as the son of man, and really man: the reasons for this: (See Gill on Matthew 16:20).