If they leave before the period of Satan's 3.5 years, how will they receive the mark?
It will be those who endure, that may not get their head chopped off soon enough. Post trib makes no sense, because all will be dead. If you are not dead, you have the mark. No way around it.
Rev 13 says that the beast "17
provides that
no one will be able to buy or to sell, except the one who has the mark,
either the name of the beast or the number of his name."
-- that means people are there who are not worshiping the beast nor do they have the mark - and they are being prevented from buying or selling. Which means the beast does not have enough access to them to behead them - so he makes it impossible for them to buy or sell.
Rev 14 contrasts the ones obeying the beast vs those who obey God. It is a warning not to obey the beast.
If anyone worships the beast and his image, and receives a mark on his forehead or on his hand, 10 he also will drink of the wine of the wrath of God, which is mixed in full strength in the cup of His anger; and he will be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb. 11 And the smoke of their torment goes up forever and ever; they have no rest day and night, those who worship the beast and his image, and whoever receives the mark of his name.” 12 Here is the perseverance of the
saints who keep the commandments of God and their faith in Jesus.
Matt 24 - does not say that "before the tribulation he will gather his elect" but rather "after the tribulation".
Rev 7
14 I said to him, “My lord, you know.” And he said to me, “These are the ones who
come out of the great tribulation, and they have washed their robes and made them white in the blood of the Lamb
Are you saying that Satan is less competent than the Romans, whom Satan was controlling all along? .
I am saying there was never a time when true Christians were not on the Earth and that even the Rev 13 statement about "no buying or selling" that is being applied to the saints at that time - indicates that they exist then as well.