Most denominations and cults deny eternal security and are working to attain salvation and denying the complete forgiveness of sins and life in Christ.
This is a good example of what I'm suggesting. WHY do they deny this? WHY are they working to gain what you and I know cannot be gained through our effort? Listen to what the person is saying and don't be side-tracked by their use of buzz words and phrases. You will hear something akin to FEAR under everything. A person will make an ISSUE of "eternal security" (either pro or con) to help cope with pressing fears. One who is born of God can easily reject the doctrine because, being subjected to Law, he cannot ignore all the sin and failure before his eyes. He is afraid to to let go of the only thing in his life that SEEMS to keep him on "the straight and narrow". On the other hand, one who is NOT born of God can CLAIM the doctrine because he hopes that if he holds on tight enough to it that he will end up in "the good place" and not in "the bad place". Another scenario: a believer will make an issue out of believing in "eternal security" because he thinks that "salvation" takes place AFTER we die (in these bodies) instead of the fact that salvation deals with real deliverance that is ours right now (and of course the "right now" never ends). Salvation is not about destiny, but about true life. It is not about believing the right things so that we can go to heaven, it's about having been made new in Christ through nothing of our own (faith).
To sum up, man without God spends his whole existence attempting to justify himself because he senses that there is no such thing as a real forgiveness or real life, so he will organize his perceptions in any way possible (especially religious) to pacify these fears and will phrase his words to make himself look as right as possible within his own little world. When those who are born of God are tricked into law-based living they will display all kinds of insanity because the life within throws them into heavy conflict. I seriously believe that those who truly doubt their salvation are those who HAVE a salvation to doubt, while those who have no life, but have plenty of religion, will desperately hold onto the "system of belief" that allows them to cope with the emptiness of their lives. Don't mistake these two opposites ... they usually are.