Then John gave this testimony: “I saw the Spirit come down from heaven as a dove and remain on him.
John again fulfills his mission of being a witness to Christ. The Greek for “I saw” is in the perfect tense which gives the verb full force; John saw with his eyes, not in a vision, the Holy Spirit actually descend from heaven to abide on Jesus, and this was not something that he saw which will soon pass away, it will have continuing effects upon John.
The Spirit “abode” (KJV) upon Jesus = the Holy Spirit began His permanent indwelling of Jesus.
The historical event is recorded in each of the other 3 gospels. Matt 3:13-17
Matt 3:13-17 Then Jesus came from Galilee to the Jordan to be baptised by John. But John tried to deter him, saying, “I need to be baptised by you, and do you come to me?” Jesus replied, “Let it be so now; it is proper for us to do this to fulfil all righteousness.” Then John consented. As soon as Jesus was baptised, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and lighting on him. And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”