The book was written about 90 - 95 A.D (CE). John was the last of the original 12 apostles alive, who had witnessed Yeshua life, miracles, death, resurrection, and ascension first hand. He also wrote the books of John and Revelation.
Paul had warned in:
Acts 20:28-31 Keep watch over yourselves and all the flock of which Ruach HaKodesh (the Holy Spirit) has made you overseers. Be shepherds of the church of G-d, which He bought with His own blood. For I know this, that after my departing will grievous wolves enter in among you, not sparing the flock. Also of your own selves will men arise, speaking perverse things, to draw away disciples after them. Therefore watch, and remember, that by the space of three years I ceased not to warn every one night and day with tears.
Paul was saying to be on guard because “wolves in sheep’s clothing” would attempt to lead the believers astray.
Only a few years had passed and Gnosticism was already beginning to take root in some of the churches/synagogues and John is writing to set the record straight on several points.
1 John 1:1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life;
Word of life - This is a reference to Yeshua and to the good news.
John 1:1 In the beginning was the Word, and the Word was with G-d, and the Word was G-d.
John 1:14 And the Word was made flesh, and dwelt among us, (and we beheld His glory, the glory as of the only begotten of the Father,) full of grace and truth.
The talmidim (disiples) lived and touched Him whom was from the beginning. John was an eyewitness of Yeshua before and after the resurrection. Yeshua was “touched” by all to testify He had a real body. The Father is spirit. The Son is the extension of the Father but is also flesh. They are One. He is light from the beginning. We also will dwell with this light and live among Him.
The first letter is Aleph - head, as in Creator, King the head of a bull appointed for sacrifice, etc.