You might be a perfect saved Christian one day, then the next day you could rape, kill, and go out on the town steaming pissed swearing your head off. Once saved definitely not always saved. It would only really matter if you were saved when you died or not. Anything you did in your earlier life, including being a saved Christian, would mean nothing if you were unsaved when you died.
Once a person is saved, is he always saved? How would you convince me if you say yes / no? It can be by the following: Illustration Bible Scriptures Reasoning Etc.
The saying: "Once saved, always saved" is a popular saying but it is not supported by the Bible. Jesus said: "he that has endured to the end is the one that will be saved", (Matthew 24:13)
*** Matt. 24:13, RS: “He who endures
to the end will be saved.” (So a person’s final salvation is not determined at
the moment that he begins to put faith in Jesus.)
Heb. 10:26, 27, RS: “If we sin
deliberately after receiving the knowledge of the truth, there no longer
remains a sacrifice for sins, but a fearful prospect of judgment, and a fury of
fire which will consume the adversaries.” (Thus the Bible does not go along
with the idea that no matter what sins a person may commit after he is “saved”
he will not lose his salvation. It encourages faithfulness.)