The assumption is to believe that Judas heard the call of Jesus, followed Him as a disciple and continued to follow Him when other discles turned away.
Correct. An assumption. And an incorrect one at that.
The scriptures also teach us this is the way we determine who believes or not by following Him.
It's one way, for sure. But not the only way. Men cannot read the hearts of others. Only God does that.
Here is the scripture -
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. 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:
The key here is v.64, where both unbelief and betrayal are mentioned in the same breath. And then in v.71 we read that the betrayer was Judas.
I don't believe that was a coincidence.
Every time believe is mentioned
So, "following Him" is mentioned every time 'believe' is mentioned, huh?
Mark 16:16 " He who has believed and has been baptized shall be saved; but he who has disbelieved shall be condemned.
Luke 8:12 "Those beside the road are those who have heard; then the devil comes and takes away the word from their heart, so that they will not believe and be saved.
Acts 4:12 "And there is salvation in no one else; for there is no other name under heaven that has been given among men by which we must be saved."
Acts 11:14 and he will speak words to you by which you will be saved, you and all your household.'
Acts 16:31 They said, “Believe in the Lord Jesus, and you will be saved, you and your household."
Rom 10:9, 10 9 that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; 10 for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation.
Rom 1:16 For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.
1 Cor 1:21 - For since in the wisdom of God the world through its wisdom did not know him, God was pleased through the foolishness of what was preached to save those who believe.
Eph 2:8 For by grace you have been saved through faith; and that not of yourselves, it is the gift of God;
2 Tim 3:15 and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus.
1 Peter 1:5 who are protected by the power of God through faith for a salvation ready to be revealed in the last time.
1 Peter 1:9 obtaining as the outcome of your faith the salvation of your souls.
2 Thess 2:13 But we should always give thanks to God for you, brethren beloved by the Lord, because God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth.
How many times does the phrase "follow Him" occur in these 14 verses that mention 'believe' or 'faith'?
For those who won't bother to count, zero is the answer.