I've read it, and the same argument many, many times.
The word believe is used of the faith at the point of regeneration and also of the faith that is a part of the Christian life. You are confusing these two.
You are projecting your dogma onto the text. Those who believe have eternal life. And eternal is eternal. But "those who believe" is not those who one day say "yes." "Those who believe" is those who live the life and persevere.
Did Paul not understand your dogma? Cause he thought that he had to persevere, work out his faith, and that he had to make up what was lacking in Christ.
SD