Why are we not to believe that Jesus was filled with the Holy Spirit after water baptism?
First of all, because that is not true. Secondly, because believing that lie may also lead people to believe other heresies, such as that Jesus only became the Lord (God) after He had been filled with the Holy Spirit; and the heresy of Jesus being reborn when He was filled with the Holy Spirit; and that Jesus had to be anointed with the Holy Spirit so that He was capable of performing His ministry on earth, etc.
Because Jesus was and is God at all times, He did not need the experience of being filled with the Holy Spirit, because He Himself is an eternal Spirit, God, who always has the power of the Spirit, and that is why He was full of the Holy Spirit of God. If you fail to understand this doctrine of the Deity of Jesus as the Bible teaches us, you are running the risk of getting lost in the maze of the kenosis doctrine.
Some people who uphold the kenosis doctrine say that when Jesus said the Son can do nothing of himself, so it is the proof that the kenosis doctrine is Biblical and correct. When we study the Bible, we see that the aforesaid thing is not true.
John 5:
15 The man departed, and told the Jews that it was Jesus, which had made him whole.
16 And therefore did the Jews persecute Jesus, and sought to slay him, because he had done these things on the sabbath day.
17 ¶ But Jesus answered them, My Father worketh hitherto, and I work.
18 Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God.
19 Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.
The Lord Jesus healed a man on the Sabbath day. The Jews persecuted Jesus, because He had healed a man on the Sabbath day. The Lord Jesus did not say that He healed this man being himself as the man or that He was not God when He healed this man. The Lord Jesus said, My Father works till now, and I work.
The Lord Jesus meant that like the Father is doing His works, so also the Son (Jesus) is doing His (the Son) works like the Father works. In other words, the Lord Jesus meant that He is God, who is doing the works of God. The Jews understood this matter exactly in that way that Jesus made Himself as God and broke the Sabbath. Jesus didn't break the Sabbath.
The words of Jesus that He can do nothing Himself, but what He sees the Father do, for what things soever He does, these also does the Son likewise means that Jesus is doing the will of His Father, but not that Jesus Himself is unable to make miracles. The Lord Jesus said the He make the things that the Father is doing. The Lord Jesus did everything by Himself, but not against the will of His Father. When the Bible speaks about the will of God, so it means that Jesus as God has the same will as His Father has.
When you understand this so you understand that Jesus' words the Son can do nothing Himself, but what He sees the Father do, means that Jesus doesn't do anything against the will of His Father, and all what Jesus did, He did it as God and He has the same will as His Father has. When the Bible says that Jesus emptied (kenoo) Himself and took on him the form of a servant, and was made in the likeness of men, and being found in fashion as a man, so it means that Jesus voluntarily went away from the glory of God's kingdom and came to be God in the fashion of a man on the earth to atone sins of the mankind. When Jesus emptied Himself, He didn't give up from His Deity, but that God became as the flesh, in other words as a man. The Kenosis doctrine is a heresy, because Jesus is in all situations God who visited on the earth in a fashion of a man.