Matt 24:15

24:15 abomination of desolation. The phrase is from Daniel; in Dan. 9:27; 11:31 it refers to the desecration of the temple by Antiochus Epiphanes. In 168 b.c. Antiochus erected a pagan altar in the temple. According to Josephus, he also sacrificed swine there. Shortly before a.d. 70 the Zealots were in the temple precincts during the war with Rome, and their presence could have been considered a desecration. In a.d. 70 the Romans entered the temple with military standards, ceremonial insignia that were elements of their religion. They took away the sacred vessels, including the lampstand, and burned the temple. Sculptures of their troops carrying the vessels are visible on the Arch of Titus in Rome.

let the reader understand. Possibly not a comment by the evangelist but by Jesus. In either case, the sense is not “whoever reads this Gospel,” but “whoever reads Daniel.”