A good comment by Thomas Lessing- "The reason why John MacArthur believes that anyone who takes the mark of the beast is saveable, is because he believes in irresistible grace. God is sovereign to save whomsoever he wills, even those who may take the mark of the beast during the tribulation because his irresistible grace is unchangeable. And why not? According to Calvinists God ordained everything that comes to pass, even the taking of the mark of the beast. Therefore, He is able and willing to impose his irresistible grace on whomsoever He wills, even on some of those (the elect) who take the mark of the beast.
On the other hand: For anyone who is going to take the mark of the beast and then repent in order to be saved, they will have to use their God-given free-will. Guess what? Calvinists do not believe that man has a free-will. How PROPOSTEROUS is putting it mildly." (end quote)
I personally believe that a false teaching like this stems from both Calvinism and the pre-trib rapture theory.