Heaven and earth shall pass away, but my word shall not pass away.

The heaven and the earth shall pass away, but my words shall in no wise pass away.

Heaven and earth will come to an end, but my words will not come to an end.

The heavens and the earth will pass away, but my words will not pass away.

Heaven and earth shall pass away,.... Which may be understood either affirmatively, as what will be at the second coming of Christ; or comparatively, rather than any thing said by Christ should; and so shows the certainty and irreversibleness of his predictions, as it follows:
but my words shall not pass away; which is true of all the words of Christ he ever spake, and here particularly relates to those he had just delivered, concerning the calamities that should come upon the Jewish nation; See Gill on Matthew 24:35.

Heaven and earth shall pass away; but my words shall not pass away--the strongest possible expression of the divine authority by which He spake; not as Moses or Paul might have said of their own inspiration, for such language would be unsuitable in any merely human mouth.
Warnings to Prepare for the Coming of Christ Suggested by the Foregoing Prophecy (Mark 13:32-37).
It will be observed that, in the foregoing prophecy, as our Lord approaches the crisis of the day of vengeance on Jerusalem and redemption for the Church--at which stage the analogy between that and the day of final vengeance and redemption waxes more striking--His language rises and swells beyond all temporal and partial vengeance, beyond all earthly deliverances and enlargements, and ushers us resistlessly into the scenes of the final day. Accordingly, in these six concluding verses it is manifest that preparation for "THAT DAY" is what our Lord designs to inculcate.