John states categorically, that no man hath seen God at any time, stating these words in his gospel account, John 1:18, and in his first epistle, 1 John 4:12.
Therefore he begins his epistle by stating what it was that he and his fellow disciples had seen.
They had seen with their eyes and they had handled the word of life. That is to say not words as such, nor mere doctrine only, but they had seen and handled the word that is of life.
Life is John's focus, here.
And he enlarges on that life.
It is 'the life the eternal' την ζωην την αιωνιον.
And it 'was with the Father'.
John points to a life that co-existed with the Father and that eternally.
Thus John states that the invisible, the divine, the eternal, the ever-living, was that which they had, in the humanity presented to them . . . 'seen' and 'handled'.
No man can see or handle deity.
But they had seen and handled 'that' which was, on earth, visible and tangible, the living presence that had ever been with the Father. One with the Father.
This is 'that' which John presents : God manifest in flesh, 1 Timothy 3:16 (TR and KJV).
Matthew Henry says as much in his commentary on this verse:
1:1-4 That essential Good, that uncreated Excellence, which had been from the beginning, from eternity, as equal with the Father, and which at length appeared in human nature for the salvation of sinners, was the great subject concerning which the apostle wrote to his brethren. The apostles had seen Him while they witnessed his wisdom and holiness, his miracles, and love and mercy, during some years, till they saw him crucified for sinners, and afterwards risen from the dead. They touched him, so as to have full proof of his resurrection. This Divine Person, the Word of life, the Word of God, appeared in human nature, that he might be the Author and Giver of eternal life to mankind, through the redemption of his blood, and the influence of his new-creating Spirit.
Matthew Henry Commentary, Biblehub
So, also, Benson in his commentary:
Now here was condescension and kindness indeed! that a person possessed of eternal, essential life, should put on flesh and blood, or the entire human nature; should assume infirmity, affliction, and mortality, in order to visit sinful mortals, to dwell among and converse with them; to reveal to them, procure for them, and then confer on them, eternal life; even felicity and glory unspeakable with himself for ever!
Benson Commentary, Biblehub
And, again also, Matthew Poole :
He interrupts the stream of his discourse by this seasonable parenthesis, while he therein gives an account how the Word of life, the life, that eternal life, ( already noted to be here all one, and chiefly to mean the Son of God), which being with the Father must be to us invisible, came to be so sensibly known to mortal men on earth; which he doth by telling us he
Matthew Poole, Commentary, Biblehub
All quotations are from the KJV based on the Textus Receptus.