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.
Jesus here defines eternal life as knowing God the Father and Jesus Christ. This 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. This intimacy with the Father is. Of course, Jesus did die to purchase forgiveness for our sins because unforgiven sins blocked us from intimacy with God. Sin was an obstacle that stood between God and us. It had to be dealt with and it was. But anyone who views salvation as only forgiveness of sins and stops there is missing out on eternal life.
Salvation was intended to be presented as the way to come back into harmony with God. Instead, it has often been presented as the way to escape the problems of this life and later the judgment of hell. It is possible to get born again with that kind of thinking, but more often than not, people who get saved through that type of ministry view the Lord as someone to help them in times of crisis and don't come to know Him.
The church has been preaching forgiveness of sins instead of relationship with God. Forgiveness of sins is an essential step to relationship with God which cannot be by- passed, but it is not eternal life. Jesus didn't die for us out of pity for our impending doom, but He died for us out of love for us (Jn. 3:16) -- a love that longed to once again have that intimate communion with man that He had originally created him for (Gen. 3:8; Rev. 4:11).
The vibrant, first century church lived relationship with God and proclaimed Jesus as the only way for others to obtain this relationship (Jn. 14:6). Without modern technology or huge sums of money, they turned the known world upside down with this gospel (Acts 17:6).
Today, the twentieth century church, with all its advantages, has made little impact on our world because we are preaching on sin and its effects instead of relationship with Almighty God. 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. They are looking for something that will fill the emptiness inside. Only intimate relationship (eternal life) with our Father can do that.