Jn 5:24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
The words of Jesus are clear, anyone who believes on him has life and shall not come into condemnation, but is passed from death to life.
The problem is that people do not know if they have believed. Why? Because faith has been turned into some magical, mystical essence that people cannot possibly understand. This is the work of the devil.
Let me ask you, do you not know if you have trusted or believed someone? For example, let's say your neighbor asks to borrow your car for an emergency. You hand your neighbor your keys and he drives off. Have you trusted your neighbor? Of course, and no one would doubt that they have trusted or believed their neighbor in this sort of example. Do you have to look inside yourself, or look for some continuous confidence to know you have trusted your neighbor? No. You know for an absolute fact you have trusted or believed your neighbor. It is an act, it is a choice you knowingly and deliberately made. It is not some magical, mystical thing.
Another example, you go to the doctor. He examines you and finds you have a brain tumor. He tells you that if not treated immediately, you will die in several weeks. Then the doctor explains that if you trust him, he can safely operate on your brain and remove the tumor. He shows you pictures of dozens of persons he has safely treated for a like condition.
So, you go in on the appointed day. You allow the doctor to put you to sleep and operate on your brain. He safely removes the tumor and in a few weeks you are completely recovered.
Now, did you trust your doctor? Of course. Is there any doubt in your mind that you trusted and believed your doctor? None. Trust is a decision you consciously and deliberately make, again, it is not some magical feeling, or this continuous confidence without doubt. You probably would have a few fears before you allowed the doctor to operate, does that negate that you trusted him and allowed him to operate on you? Not at all.
People in the scriptures KNEW when they had believed.
Acts 19:1 And it came to pass, that, while Apollos was at Corinth, Paul having passed through the upper coasts came to Ephesus: and finding certain disciples,
2 He said unto them, Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost.
3 And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism.
4 Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.
5 When they heard this, they were baptized in the name of the Lord Jesus.
Notice Paul's question in vs. 2, he asked whether these disciples had received the Holy Spirit since they believed. Did they say, "We don't know, we don't know what it means to believe, how do we know if we have believed?". No, they KNEW they had believed.
Believeing is a committal, it is trusting your soul into Jesus's hands and depending on him to save you, just as you trust a doctor to operate on you, you place your life in his hands.
There are many ways believeing is described in scripture. It is described as "coming" to Jesus. If a person comes to Christ in their heart desiring that he save them, this is trust, this is believing. The scriptures describe believeing as "calling" on Jesus. Whosoever calls upon the name of the Lord shall be saved. When you cry out to Jesus, you have placed the matter of your salvation completely in his hands, you are depending or relying on him only to save you.
Can you know if you have cried out to Jesus in your heart to save you? Of course. It is not some magical mystical thing, it is a decision of the will to trust or depend on Jesus alone for salvation.
It is these folks that make faith into some mystical essence that has folks all mixed up. You can KNOW if you have believed on Jesus, and if you have, you have everlasting life according to God's word.