This prologue parallels the prologue to the fourth gospel. But whereas the Gospel emphasizes the deity and eternality of the Word (logos), the epistle lays its stress on the full humanity of the Word that became flesh and thus is the Word of life. Hence the Word both embodies and conveys the eternal life that is the basis of fellowship (koinonia) with the apostles (us”) and with God (v. 3).
The key theme of 1 John is eternal life. As in the fourth gospel eternal life does not primarily refer to unending existence as a future hope, but to the “life of the age to come” that has become a reality in the present. The NT writers all declare that the age to come has broken into the present age in the person and work of Jesus Christ (see Ladd, 45ff.). Thus to be united with him is to experience this life here and now.
The paragraph opens with a series of relative clauses, piled up to emphasize the reality of the Word become flesh. He was no mere appearance, as false teachers apparently insisted, but was subject to the empirical senses. He could be heard, seen, and touched: “to have handled was the conclusive proof of material reality” (Stott, 59).
That which was from the beginning here refers to the beginning of the Gospel, not the beginning of the universe (as Jn 1:1). John proclaims his own knowledge of it from personal contact with Jesus Christ.