Does The Christian's Present Possession Of Eternal Life Prove "Once Saved Always Saved"?

It is true a faithful Christian presently possesses "eternal life." That is indisputably proven by passages like John 5:24, 6:47, 6:54, I John 5:11-13, John 3:36 ("He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him"), and I John 3:15 ("Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him"). But that fact doesn’t prove the doctrine of "Once Saved Always Saved." Illustration: In the very beginning Adam and Eve possessed eternal physical life (Gen 3:22), but they lost it when they sinned. Likewise God can give someone eternal life and then they can forfeit it (Isaiah 59:2). It is possible to possess something (in this case, spiritual life) that lasts forever, but not to possess it forever. For example, I own a mortgage that is to last 30 years, but I would be glad to sell it to you for a fair price - then I wouldn't possess it anymore. See how the loan lasts 30 years, but I don’t necessarily possess it for 30 years? Similarly God gives us eternal life (as opposed to physical, temporary life) when we become His disciple (John 5:24), but that doesn't necessarily mean we can't lose that spiritual life due to our sin (Romans 6:23). The Bible most certainly does teach we can "fall from grace" (Galatians 5:4, II Peter 2:20-22, etc.).