Oh yeah? A convincing argument convinces at least those that are making it. I see evidence all the time that Protestants do not actually believe it themselves. This is shown in their concern that some of their fellow Christians, presumably in a state of grace by accepting Christ, are being led astray by new age thinking, near-death experiences, Mormonism, liberalized "progressive" Christianity, Satanism, and of course the biggie Catholicism. Why the concern if you cannot fall out of grace? Are they not going to heaven anyway by the mere fact they at one time accepted Jesus Christ as their Savior?
Well, I’m pretty convinced by it. The preaching of the Gospel is the means by which God calls out His elect. What greater joy is their than to be used by God to speak the truth? Especially the truth against the biggie, known as Popery? But as for those who leave us, it is as John said. They must leave us to show they were not of us. If they were of us, they would have remained with us. It is impossible, as Jesus said, for the elect to be lost, even in the face of incredible deception. I believe Him. You should too.