Because ever = at any time or always; and lasting = means to endure, except this is in the present continuous tense.
So it does in fact define itself. It's not an opinion, words do that - it's how language works, and the fact the Lord uses this particular description for our salvation defines our life as always enduring, or everlasting.
This doesn't even make sense. Show me one scripture where it says that a believer who is saved can lose their salvation and be condemned to hell. You have to prove what you believe based upon scripture just as much as I do.
Here's what proves everlasting life as the true, biblical doctrine:
To enter the kingdom of heaven, we must be born again, or in other words, born of God (John 3:3). To be born again, we must believe that Jesus is the Christ and Son of God (1 John 5:1). When we are presented the Gospel, receive and believe it, we call upon the name of the Lord in faith, believing in the heart and confessing with the mouth the Lord Jesus (Romans 10:9, Romans 10:10, Romans 10:13, Acts of the Apostles 16:31, Acts of the Apostles 2:21), then we are sealed with the holy Spirit unto the day of redemption (Ephesians 1:13), and we are promised everlasting life by the words of Jesus (John 3:16) and pass from condemnation and death, unto justification and eternal life through Christ (John 5:24, John 3:18, John 10:28-29), whereby we know we are secured in the grace of God by faith having been reconciled to him as the sons of the living God (Romans 5:1-2, John 1:12-13, Romans 5:10-11), and are confident that nothing can separate us from the love of God through Christ (Romans 8:38-39, Hebrews 13:5), because know that God cannot lie when he made the promise that all who receive eternal salvation have an eternal inheritance with him through Jesus Christ (Titus 1:2). For if God unsealed us and made us unborn from him after already having been born, and took away our everlasting life, God would be found a liar, because he promised us salvation through Christ by faith; but God is not a liar, as you make him out to be.
Once saved, always saved.