As we know, John and his brother James, along with Peter, were the closest to Jesus.
John, along with James and Peter were the sole witnesses to Jesus's transfiguration on the mountain.
It may have been this John that dictated the Gospel of John and some scholars believe that there was John the Apostle, John the elder and John of Patmos, that all were different John's.
Well, lets not go there right now, but in regards to John of Patmos being John the apostle:
John the aspostle saw Jesus's transfiguration:
2 Six days later, Jesus took with him Peter and James and John, and led them up a high mountain apart, by themselves. And he was transfigured before them, 3 and his clothes became dazzling white, such as no one l on earth could bleach them. 4 And there appeared to them Elijah with Moses, who were talking with Jesus. 5 Then Peter said to Jesus, “Rabbi, it is good for us to be here; let us make three dwellings, m one for you, one for Moses, and one for Elijah.” 6 He did not know what to say, for they were terrified. 7 Then a cloud overshadowed them, and from the cloud there came a voice, “This is my Son, the Beloved; n listen to him!” 8 Suddenly when they looked around, they saw no one with them any more, but only Jesus.
And yet, John of Patmos, when he saw Jesus like that, needed Jesus to explain to him who he was:
13 and in the midst of the lampstands I saw one like the Son of Man, clothed with a long robe and with a golden sash across his chest. 14 His head and his hair were white as white wool, white as snow; his eyes were like a flame of fire, 15 his feet were like burnished bronze, refined as in a furnace, and his voice was like the sound of many waters. 16 In his right hand he held seven stars, and from his mouth came a sharp, two-edged sword, and his face was like the sun shining with full force. 17 When I saw him, I fell at his feet as though dead. But he placed his right hand on me, saying, “Do not be afraid; I am the first and the last, 18 and the living one. I was dead, and see, I am alive forever and ever; and I have the keys of Death and of Hades.