John 17:3 'And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.'
In order to fully understand what eternal life is, it is helpful to understand what it is not. Eternal life is not living forever. Everyone lives forever in either heaven or hell. Also, eternal life is not living forever in the blessings of heaven as opposed to being tormented in hell.
John 3:36 and 5:24 show that eternal life is a present tense possession of the believer. Here, Jesus defines eternal life as knowing God the Father and Jesus Christ.
As we have already discussed, the word 'know' is speaking of intimacy instead of mere intellectual knowledge. Therefore, eternal life is having an intimate, personal relationship with God the Father and Jesus the Son.
According to John 3:16, this intimacy with God is what salvation is all about. Forgiveness of our sins is not the point of salvation: intimacy with the Father is. Of course, Jesus did die to purchase forgiveness for our sins because unforgiven sins block us from intimacy with God. Sin was an obstacle that stood between us and God. It had to be dealt with and it was. Anyone who views salvation as only forgiveness of their sins and stops there, is missing out on eternal life. Salvation is intended to be the way to come back into harmony with God. Instead, it is often presented as the way to escape the problems of this life and the judgment of hell later.
Most non-believers are so occupied with their 'hell on earth' that they don't really think or care about their eternal future. They are fed up with religion and are looking for something that will fill the emptiness inside. Only intimate relationship (eternal life) with our Father can do that.