For God so loved the world that He gave his one and only Son that whoever believes in him shall not perish but have eternal life.
Some emphasize the fact that eternal life is eternal. It is claimed that if we could forfeit salvation, eternal life would then cease to be eternal. This fails to recognize the important truth that there is no eternal life outside of Christ (John 1:4; 5:26; 6:35; 11:25; 14:6; 1 John 1:2; 5:11; Col. 3:3, 4; 2 Pet, 1:4), and we share in his life only as we remain in Him through saving faith (those “believing” in this passage). Eternal life does not cease to be eternal if we fail to continue in saving faith, we will simply cease to participate in the eternal life which resides only in the Son of God. Eternal life will continue on just fine without us, if we fail to meet the condition of faith.
Robert Picirilli comments:
Those passages, especially in the Gospel of John, which contain strong promises of (final) salvation to believers and are therefore thought to imply necessary perseverance can not be used for that purpose lest they ‘prove too much.’ In other words, to say that those promises require the impossibility of a changed situation places too great a burden on the syntax of the statements. And this can quickly be seen by comparing similar promises, using the very same syntax, to unbelievers. For example:
He that believes shall not come into condemnation.
He that believes not shall not see life.
Grammatically, if the first means the condition of the believer can not be changed, then the second means the condition of the unbeliever likewise can not be changed. In fact, neither passage is even speaking to that issue. The unbeliever can leave his unbelief, become a believer, and see life- thus escaping from the promise made to the unbeliever who continues in his unbelief. Likewise, the believer can leave his belief, become an unbeliever, and come into condemnation- thus escaping from the promise made to believers who continue in faith. Each promise applies with equal force to those who continue in the respective state described. (Grace, Faith, Free Will, pp. 200-201)