147 Matthew continues in the following strain: “And as He sat upon the mount of Olives, the disciples came unto Him privately, saying, Tell us, when shall these things be? And what shall be the sign of Your coming, and of the end of the world? And Jesus answered, and said unto them, Take heed that no man deceive you: for many shall come in my name, saying, I am Christ; and shall deceive many;” and so on, down to where we read, “And these shall go away into everlasting punishment, but the righteous into life eternal.”
We have now, therefore, to examine this lengthened discourse as it meets us in the three evangelists, Matthew, Mark, and Luke. For they all introduce it in their narratives, and that, too, in the same order. Here, as elsewhere, each of these writers gives some matters which are peculiar to himself, in which, nevertheless, we have not to apprehend any suspicion of inconsistency. But what we have to make sure of is the proof that, in those passages which are exact parallels, they are nowhere to be regarded as in antagonism with each other.
For if anything bearing the appearance of a contradiction meets us here, the simple affirmation that it is something wholly distinct, and uttered by the Lord in similar terms indeed, but on a totally different occasion, cannot be deemed a legitimate mode of explanation in a case like this, where the narrative, as given by all the three evangelists, moves in the same connection at once of subjects and of dates. Moreover, the mere fact that the writers do not all observe the same order in the reports which they give of the same sentiments expressed by the Lord, certainly does not in any way affect either the understanding or the communication of the subject itself, provided the matters which are represented by them to have been spoken by Him are not inconsistent the one with the other.
Source: The Harmony of the Gospels (New Advent)