[1:1] That which was from the beginning, which we have heard, which we have seen with our eyes, which we looked upon and have touched with our hands, concerning the word of life— the life was made manifest, and we have seen it, and testify to it and proclaim to you the eternal life, which was with the Father and was made manifest to us— that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ.  And we are writing these things so that our (or your) joy may be complete. (1 John 1:1-4 ESV)
In this opening paragraph St. John clues us in to several key themes of his epistle, but the special burden seems to be that of life. And at this early point it is wise to consider what it is that John has in mind when he speaks of life, and particularly eternal life, as he will a number of times throughout his letter. His first reference is to the Word of Life, which is a clue. Jesus Christ is himself the eternal Word who came offering life (John 1.1), and life abundantly. Immediately in verse 2 we are told that the Life was made manifest, confirming our suspicion that the Life/Word of Life referred to is in fact Jesus himself and the life that spills out of him like living water. It is Jesus and his overflow of life (in contrast to the spreading death that has been with us since the fall) that John and his compatriots have heard, seen and touched.
But this leads us to the more specific issue of "eternal life," (v. 2b) a term which will turn up numerous times in John's epistle. It deserves some reflection at the outset. My guess, based on my own experience, is that most Christians tend to kind of read over this quickly assuming that it basically refers to something like living forever with God after we die. However, I think we need to slow down a little and consider what John may have meant. In the first place there is the general problem with the common conception of salvation in evangelical circles which tends to say something like this: when we die we go up to heaven and take on an ethereal life of disembodied worship of God for eternity. Suffice it to say that while there is no time to refute such a view here, that is not a Biblical picture. Eternal life is embodied life, resurrected life after the pattern of our savior and elder brother, Jesus Christ who was raised bodily and ascended into heaven bodily, and sits at the right hand of the Father bodily to this day and forevermore. (There is a man in heaven.) But I believe there is an even more specific nuance here. As John uses and develops the notion of eternal life it seems to me that he is hinting at quality as much or more than he is at quantity; that is, his point is as much about the inheritance of a fulness of life as it is about unending life simpliciter. And this is very much within the lexical/semantic range of the word we typically render 'eternal' in English.
I don't want to be too pedantic here, but what I am suggesting is that John's primary point when he talks about eternal life is that as the Word of God incarnate, Jesus Christ, came into the world he brought with him the very life of God, the life of the Trinity, which is itself eternal; that is, both everlasting in a temporal sense, and full and complete in a qualitative sense. It is this full and rich and abiding life that John and his fellow apostles have experienced right here in this embodied life (heard, seen, and touched) and that they now proclaim, and in doing so share with their hearers. And the motive for such proclamation is to bring his readers/hearers in. By having experienced this fullness of life, the very life of God manifested in the incarnate Son, John and his co-laborers have been taken up into the life of the Triune God and indeed function as extensions of that life. Thus, to bring others into fellowship with themselves is to bring those same into the divine life of the Trinity. (Wonder of wonders this is true for us too, and ought to serve as a prime motivator for evangelism: we are united to Christ and thus bound up in the wellspring of life, the life of God, and as we share the gospel we invite other into that very life.)
It is this conviction: that because of our acceptance and love for the living, loving, incarnate Son of God we are bound up in the everlasting life and love of the Holy Trinity, that I think shapes the whole of St. John's first epistle.