When a replacement for Judas was chosen by the Apostles a requirement was that he be someone who had been with them from the beginning. This particular office , that is, one of the twelve, required that one had been a with them since the beginning. St John stesses the physicality of the Apostolic witness. That witness is the most intimate possible on earth. It’s that particular witness that the most powerfull heresies attack. The Witness establishes a teaching authority that is recognized by the Bereans for example. The unique and exclusive witness of the original 12 possesses because of that original, physical, material, witness of God on earth a knowing of the Truth , Way, and Life and consequently a teaching authority that no other can take away or possess without it being handed down physically, materially. St Paul experienced a first hand witness of Christ in vision and this witness also makes one an apostle but as scripture reveals it is subordinate too and subject to the judgement of the Witness of those who share in the original Bond established by Jesus, Confimed by the Father in Peter and strengthened by the Holy spirit at Pentecost.