Romans 8:38 and Eternal Security

Here's a link to a message by Phil Johnson on eternal security. I will admit that the fifth point of Calvinism - the perseverance of the saints - remains the one of the five that I have the most trouble with. Even election I'm fine with, but the teaching that no genuine believer can ever leave the faith is one I do find myself wrestling with from time to time, especially when you know of people who, once vibrant in the faith, have let it lapse. Maybe the genuine faith wasn't there to begin with (1 John 2:19) but it still remains one of those I toss around in my head. Can one be a 4.5-point Calvinist? :)