1 John 1:1-4
Notice the wording carefully. What is it that John says was manifested, that they experienced with their own senses? Eternal life! Eternal life is something that in the biblical sense can be seen and heard. Indeed, the apostles fellowshipped with it in the flesh! In turn, they reported it to us so we can also fellowship with it - though not to the same extent and in the same manner as they did.
Of course, John is speaking of witnessing and fellowshipping with that kind of life as exemplified in Jesus Christ. Verse 3 is the specific purpose statement of this epistle of I John: to proclaim the reality of God's eternal life as revealed in Jesus Christ.
When John wrote this epistle, the Gnostic heresy was rising in the church. We should note that John's method of countering it is highly subjective, that is, the epistle has many references to the first-person pronouns "I" and "we." The apostle uses the weight of his personal experience witnessing this life to combat the heresies of the Gnostics.
He says the life we witnessed "was from the beginning"; it is the original manner of living. It is the ultimate reality of how to live. This kind of life is not subject to change, whether over time or from culture to culture. The ultimate reality is God - in this case Jesus Christ in the flesh, who is God - and He changes not.
John W. Ritenbaugh
The Elements of Motivation (Part Six): Eternal Life