What are believers in Judea supposed to do when they see the abomination of desolation?

When the abomination of desolation spoken of by Daniel the prophet appears, and the man stands in the holy place, those in Judea are to flee to the mountains. This occurs at the midpoint of the seven-year tribulation, after three and a half years.

Les explains that Matthew 24:15 is Jesus speaking to the twelve on the Mount of Olives after they asked about the signs of His coming and the end of the age. Verses 1–14 describe the opening of the seven years—famines, wars, pestilences, and so forth—but verse 15 brings them to the midpoint of that tribulation. The abomination of desolation is spoken of by Daniel the prophet in Daniel 9:26–27. When that man stands in the holy place, and the reader understands, then those who are in Judea are to flee to the mountains.

Matthew 24:15

When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:)

Who is the abomination of desolation that Daniel predicted?

The abomination of desolation Daniel predicted is the Antichrist, who at the midpoint of the seven-year tribulation moves into the temple in Jerusalem and stands in the holy place. Jesus referenced Daniel's prophecy, confirming Daniel as a prophet. When this happens, those in Judea must flee to the mountains.