1:1 “what we proclaim to you” is the subject of verses 1-3
1:1 John (the apostle) doesn’t just proclaim what was from the beginning but proclaims what he personally heard, saw, and touched. So Jesus dying and raising from the dead wasn’t just some “mystical experience” or word of mouth. The apostles (John refers to the other apostles in 1:1 with the usage of “we”) actually proclaim they saw, heard, and touched this reality. Or, as John puts it in 1:1 and 1:2, the “life.”
1:1 and 1:2 John uses “life” again to refer to Christ like he did in the Gospel of John
1:2 This “life” was revealed (Jesus) and was eternal. John here proclaims Christ’s deity. Jesus wasn’t mere man. John proclaims more than hearing, seeing, touching the man Jesus. He heard, saw, touched God.
1:3 John announces “what they have seen and heard (i.e. Jesus) so that (i.e. purpose) the readers may share in what the apostles have: fellowship with the Father and Son.
1:4 The purpose of writing the readers is so John’s (along with the other apostles) joy may be complete.
Putting it together
Reading Lenski (Commentary on the New Testament), it seems there’s a link between the readers having fellowship with the apostles and the message of the apostles. In other words, having fellowship with the apostles means accepting their teaching. Those who reject this teaching have no relationship with the Father and Son. According to Lenski, this is pivotal in the book of 1 John because 4:15 will say “Whoever confesses that Jesus is the Son of God, God remains in him and he in God.” And this is the fellowship with God.
Practical take away
If you reject the message of the apostles (that Jesus died and rose again), you don’t have a relationship with the Father and Son. Don’t believe the lies of this world! The apostles preached the truth and experienced it. We can believe them.
This is meant to help teach layman how to study the Bible without Greek/Hebrew training. Excelling in simple observation skills greatly enhances personal study time and more accurately reveals correct interpretation by noticing the immediate and broader context.