Vincent's NT Word Studies
21. He straitly charged (epitimhsav). The word implies an emphatic, solemn charge; its meaning being, strictly, to lay a penalty upon one, and thence, to charge under penalty.No man (mhdeni). The conditional negative: no man, whoever he might be.
22-27. Compare Matt. xvi. 21-28; Mark viii. 31-38; ix. 1.
Robertson's NT Word Studies
9:21 {To tell this to no man} (medeni legein touto). Indirect command with the negative infinitive after {commanded} (pareggeilen). It had been necessary for Jesus to cease using the word {Messiah} (cristos) about himself because of the political meaning to the Jews. Its use by the disciples would lead to revolution as was plain after the feeding of the five thousand (#Joh 6:15).