Is Salvation really "eternal"?
The "being born again" idea Jesus talks about in Jn. 3 is addressing the issue of "seeing" and "entering" into the Kingdom of God but He didn't answer Nicodemus questions: How can these things take place or be?
This man, a teacher who supposedly knew the Scriptures in and out, had missed the spiritual truths that Jesus was trying to present to him but in the answer there are some concepts that the "natural man" would fail to understand. I suspect Nicodemus had enough light in him (demonstrated by his visiting Jesus) to receive the good seed.
But all in all, if Salvation if free, do people need to do something in order to get it? If salvation is eternal, does it really have a starting point? Is physical death a spiritual event or just another step in the Christian progression?