Luke 21:20 KJV
And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh.21Then let them which are in Judaea flee to the mountains; and let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto.
Mark 13:14
But when ye shall see the abomination of desolation, spoken of by Daniel the prophet, standing where it ought not, (let him that readeth understand,) then let them that be in Judaea flee to the mountains:And let him that is on the housetop not go down into the house, neither enter therein, to take any thing out of his house:
These are two different signs. Jerusalem compassed with armies (in the physical world) is not an abomination, it's judgment. “standing where it out not” can't describe the armies because the armies were put there by God for the purpose of judgment. We simply can't conclude that these two versus are the same thing.
They don't match unless we give Luke's version spiritual meaning, then they match perfectly...
Spiritually speaking, Luke's “compassed with armies” means this:
Revelation 20:9-11
9And they went up on the breadth of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them.
Luke's Jerusalem being compassed with armies (in the physical world) is a demonstration of a spiritual emanation which took place in the garden of Eden. It is symbolic of “abomination that causes desolation” because no unclean thing can touch or enter the temple, or Holy city of God. The armies represent filthy evil doers (such as the pharisees and their spineless, brainwashed slaves)who try to enter the temple and be one in spirit with God. This is an abomination that will lead to a person's spiritual desolation and destruction. Jesus was giving them a head's up to prevent them from getting caught in the spiritual cross fire, which no sheep is immune from if he or she backslides, or returns to the field, or vomit, like a dog.