148 Again, what Matthew states in this form, “And this gospel of the kingdom shall be preached in all the world for a witness unto all nations, and then shall the end come,” is given also in the same connection by Mark in the following manner: “And the gospel must first be published among all nations.” Mark has not added the words, “and then shall the end come;” but he indicates what they express, when he uses the phrase “first” in the sentence, “And the gospel must first be published among all nations.” For they had asked Him about the end. And therefore, when He addresses them thus, “The gospel must first be published among all nations,” the term “first” clearly suggests the idea of something to be done before the consummation should come.
Source: The Harmony of the Gospels (New Advent)