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— (1 John 1:1 ESV)
The author of 1 John starts off by speaking about a subject, a "that" which was from the beginning.
Further on in the passage it is made clear that the "that" being spoken of is none other than Jesus Christ. So lets breakdown what was said about him line by line.
"That which was from the beginning.."
Here, Jesus is said to have always been and never began. This is to defend His divine nature which at that time was being both questioned and distorted. Only Yahweh (God) is eternal, so to say that Christ had no beginning is to call Him God.
"Which we have heard..."
The original apostles, the hundreds of followers, the thousands of people whom listened to His teachings- they heard His message. They remembered the way He spoke with authority. This is said to remind believers that the message Christ spoke was heard and there are witnesses to it, whom can vouch for the Gospel that is now being preached- as what Christ spoke.
"Which we have seen with our eyes, which we have looked upon and touched with our own hands.."
Here he speaks about the fact that they saw Jesus for who He said He was, more importantly, that they saw Him risen from the grave, touched Him, looked upon him, and believed.
"Concerning the word of life.."
Christ is called "The Word" in John 1 , and calls Himself, "The Life". Understand that:
1. Through God SPEAKING, was creation and LIFE formed. God's Word, Christ, brings LIFE because He is LIFE
2. Christ is the only way to eternal life. There is NO OTHER WAY. If He is life, it is illogical to believe that you can live outside of Him.
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— (1 John 1:2 ESV)
"The life was made manifest, and we have seen it.."
Life manifested itself to all of creation. Christ took the form of human flesh and made Himself known to all mankind.
The incarnation of Christ was, and still is, a doctrine that is under constant attack. Here John says, "God came down, in human form, AND WE SAW IT"
"And testify to it and proclaim to you the eternal life.."
So because they saw Christ, abided with Him, listened to Him, and believed Him- they can testify about Him.
They proclaim the eternal life by preaching Christ- who IS THE ETERNAL LIFE.
"Which was with the Father and was made manifest to us.."
Jesus was with the Father. Jesus was with God in Heaven, seated at the right hand, in all glory and power and decided to come down to earth, to a spiritually dead creation, and bring forth LIFE.
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. (1 John 1:3 ESV)
"that which we have seen and heard we proclaim also to you, so that you too may have fellowship with us"
Here the author states that they share the Good news so that people may be part of the fellowship of the saints. They had seen and believed that Jesus was the Son of God, that He was God. This is the statement that defines those whom are in the fellowship of the saints, and those whom are not.
"and indeed our fellowship is with the Father and with his Son Jesus Christ."
Our fellowship is with God and Jesus. Father and Son. We are not only in union with Christ through His death an resurrection, but in communion with the Father, Son, and Holy Spirit. You CANNOT be in fellowship with the Father, if you do not believe that Christ is God.