A.T. Robertson Commentary Luke 21:27

"And then shall they see the Son of man coming in a cloud with power and great glory." — Luke 21:27 (ASV)

And then shall they see (κα τοτε οψοντα). As much as to say that it will be not till then. Clearly the promise of the second coming of the Son of man in glory here (Mr 13:26f.; Matthew 24:30f.) is pictured as not one certain of immediate realization. The time element is left purposely vague.