John the Baptist was seeking the Lamb of God, and at the same time building a group of disciples and followers for the Lamb when he was found.
He did this by baptism; which was a 'diagnostic test', a supernatural method for discernment.
When John baptized, he could see the movement of the divine spirit. When he saw the spirit descend and touch upon a person during baptism, this person was accepted as a disciple.
These disciples would then bring other people to John for "testing".
The author of the Fourth Gospel (miscalled "John" - actually Lazarus) was one such disciple, and brought his sister's betrothed husband Jesus to John for testing.
When Jesus was baptised, the divine spirit descended and stayed with Jesus. By this, John knew that Jesus was the Lamb of God; and Jesus became fully divine - such that he could consciously participate in creation and perform miracles.
John, and also Jesus's chosen disciples, continued to recruit followers by discerning-baptism for a while; but John's essential work had been done by finding Jesus and being the agent of his divinization - thus John's ministry waned as Jesus's waxed.
Note: To evaluate the above, you might try reading the earlier chapters of the Fourth Gospel, bearing in mind this "diagnostic"/ discerning explanation of the nature of baptism.