The question that must be asked here is this: "How long does 'eternal life' last?" Well, by definition, "eternal life" lasts forever. On this basis alone one can confidently reject the notion that our salvation, our "eternal life", can be forfeited. If our "eternal life" can come to an end, then it is not "eternal life." If "eternal life" is anything but "eternal" and "everlasting" then Jesus has given false promises. Eternal life means eternal life.
Someone might avoid the above conflict by asserting that eternal life doesn't begin until we die physically. But is this true? Well, IF it's true, then your eternal life is conditioned not on your faith in Christ, but on what you do between the time you believe in Christ and the time you die. In other words, salvation becomes a product of your works and not a product of God's grace through faith in Christ.
John 3:3, John 3:7 and 1 Peter 1:23 all use the phrase "born again," referring to salvation. John 3:6 contrasts the birth of the body (flesh) against the birth of the spirit. Also, 2 Cor 5:17 and Gal 6:15 both say that you are a "new creature" in Christ. If our eternal life doesn't begin until we die, then why did the Holy Spirit use these descriptions?
Eternal life begins the moment you place your trust (faith alone) in Jesus Christ for salvation. This is when you become a "new creature" and are "born again." If eternal life begins at salvation, and if eternal life is truly "eternal", then there can be nothing we can do to lose our salvation.
The following verses all promise eternal life or everlasting life specifically, or "life" synonymously, conditioned on faith in Christ: John 3:15, John 3:16, John 3:36, John 5:24, John 11:25, John 20:31, 1John 5:13 And note that John 5:24 and John 11:25 both represent the words of Jesus Christ Himself.