I’m going to throw a monkey ranch into this discussion, then duck in hopes not everything thrown will succeed in hitting me.
The explanation about “He must increase, but I must decrease” that sounds the most reasonable to me - (though not necessarily the most likely!) - is the one offered by the Unification Church (Moonies), now known, I think, as the Family Federation. Their explanation is that the centrality of the Will of God was leaving John and going to Jesus. Not that it had to. I’m not sure how well I can explain it without the “Divine Principle” (The Moonies’ scripture thought they usually deny it is scripture) in front of me.
They argue that if John had stuck with Jesus, John would have become the Chief Disciple, the First Apostle, and shared in the glory and honor that Jesus received. Because John’s faith in Jesus faltered (they don’t say this, but I’m sure it is basically what they mean), he did not follow Jesus. Because of human pride (!), John continue with his own ministry, keeping his own disciples with himself rather than telling them to follow Jesus. Not instructing his disciples to switch their allegiance to Jesus, is the strongest evidence for the Moonie position, imo.
Because the Providence of God centered around Jesus, whoever was with Jesus - the 12 Apostles - would share in the blessings of that Providence, and would be recognized as foremost Christians. Whoever distanced himself from Jesus, would not be part of that Providence, and would not be recognized as a great Christian. Although I don’t think it says this in the Divine Principle, the only testimony John gave about Jesus was the alleged saying at the waters where John was baptizing, “Behold the Lamb of God.” For all I know he could have said that to a number of people, part of a baptismal or post-baptismal liturgy; or he could have been deriding Jesus for taking a pacific approach to the sins of the king while John was preaching hell-fire and putting his life at risk - John was being a lion before the adulterous king, while Jesus was being a submissive lamb before both the Jewish and the Roman authorities. (I don’t mean Jesus didn’t escape from them when his life was at risk. What I mean is that he didn’t condemn them as directly and as harshly and as relentlessly, and as particularly as John was doing with Herod. And yes I recognize that
later, Jesus did take up arms to throw off the oppression of the Establishment.)
I know many Christians take John’s saying, “He must increase, but I must decrease,” as a witness to Jesus as messiah or prophet or something. That would be a very vague, ambiguous, even silly sort of “testimony”. It isn’t a testimony at all! Herod, too, “must increase” while John “must decrease”. If the quote is literal, it’s not clear what is changing size. If it’s metaphorical, it still isn’t clear.
That John the Baptist is in fact considered the greatest testifier to Jesus, suggests to me that John’s disciples were effective in promoting John’s message, and John himself as a holy man. And probably that Jesus’ disciples had no interest in belittling John. Jesus and his disciples probably respected John a lot, not matter whether he was going off to do his own mission, or was sitting around the campfire with the others, roasting fish and hanging on every word of the Messiah.
