These verses form a refrain from a song of the 60's or 70's, "I wish we'd all been ready." The assumption of that song was that this could happen any moment, but the thrust of this passage (Matt. 24:32-35)is that it will happen in their generation.
Jesus connects the certainty of these things with the truthfulness of His word. His word is true. . .therefore they can count on these things happening in their generation.
The celestial cataclysm is the piece of this chapter that makes many think that these events are still to happen and make it hard to understand in the past tense.