Lit., thou shalt be being silent. The finite verb and participle denote continuance.
Not able to speak
Showing that the silence would not be voluntary.
My words which (οἵτινες)
The pronoun is qualitative, denoting a class. “My words, which, incredible as they seem to you, are of a kind which shall be fulfilled.
In their season (εἰς τὸν καιρὸν)
The preposition implies exactness: at the completion of the appointed time. The process of fulfilment, beginning now, will go on, εἰς, up to, the appointed time, and at the time will be consummated. Καιρὸν, season, is more specific than χρόνος, time. It is an a appointed, fitting time: the right point of time when circumstances shall concur.