We know he was a thief who helped himself to the money bag.
He did not say this because he cared about the poor but because he was a thief; as keeper of the money bag, he used to help himself to what was put into it.
John 12:6
He was an opportunist. Yes, he saw many miracles. So did the Pharisees. It just made them jealous and mad.
Are you saying that no believer ever stole anything?
Does your theology teach it is impossible for a believer to steal, or lie or commit adultery.
Here Paul warned the Church at Galatia, believers in Jesus Christ, more than once if they continued to practice the works of the flesh, they would not inherit the kingdom of God.
19 Now the works of the flesh are evident, which are: adultery,fornication, uncleanness, lewdness, 20 idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, 21 envy, murders, drunkenness, revelries, and the like;
of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
Galatians 5:19-21
Your claim that Judas wasn't a believer because he took money from the money bag, holds no weight in the light of scripture.
The scripture says, that the apostles, including Judas, believed Jesus was the Christ, the Son of the living God, in which they all continued to follow Him, when the other disciples turned back from following Him, proving that they continued to believe.
64 But there are some of you who do not believe.” For Jesus knew from the beginning who they were who did not believe, and who would betray Him. 65 And He said, “Therefore I have said to you that no one can come to Me unless it has been granted to him by My Father.”
66 From that
time many of His disciples went back and walked with Him no more. 67
Then Jesus said to the twelve, “Do you also want to go away? ”68 But Simon Peter answered Him, “Lord, to whom shall we go?
You have the words of eternal life. 69 Also we have come to believe and know that You are the Christ, the Son of the living God.” John 6:64-68
Your opinion, doesn't line up with what the scripture teaches us.
JLB