Mark 13:33
Be on guard! Be alert! You do not know when that time will come — the third be on guard command in the discourse (verses 5, 23, 33). The three-fold warning creates a sustained refrain of vigilance: the response appropriate to the Olivet Discourse is not chronological calculation but constant readiness. You do not know when: the unknown timing is not a frustration but the theological basis for the vigilance command — if you knew exactly when, vigilance would be unnecessary.