First off, just because some people misapply the doctrine of eternal security does not make it any less true. Those who would believe they can live any ol' way they wish and still be saved have yet to actually be saved. They have not repented.
Secondly, the version of the Bible you're quoting from is incorrect. More reliable versions say: "For there shall arise false Christs, and false prophets, and shall show great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect."
"if it were possible" inplies that the deception is going to be so strong that "if it were possible" even the elect would be deceived. But it's not possible. This is a literary usage to make a point.
This is what John Gill in his commentary said:
IF IT WERE POSSIBLE:
"But by the elect are meant, a select number of particular persons of Adam's posterity, whom God, of his sovereign goodwill and pleasure, without respect to their faith, holiness, and good works, has chosen, in Christ, before the foundation of the world, both to grace and glory: and to deceive these finally and totally, is impossible, as is here suggested; not impossible, considering their own weakness, and the craftiness of deceivers, who, if left to themselves, and the power of such deception, and the working of Satan with all deceivableness of unrighteousness, might easily be seduced; but considering the purposes and promises of God concerning them, the provisions of his grace for them, the security of them in the hands of Christ, and their preservation by the mighty power of God, their final and total deception is not only difficult, but impossible."
This verse actually bolsters the doctrine of eternal security! It does nothing to undermine it. More modern versions have taken out "it where" from the statement "if it were possible"... to make it "if possible". The first one is correct, and has always been translated that way until sometime in the 1900's. Not only is this new translation not true to the original language, but it also contradicts Bible doctrine, and other verses such as John 10:28,29.
Krispy