The late Dr. J. Vernon McGee had a radio broadcast called Through the Bible. The radio broadcast was centered on a five year, verse-by-verse, study through the entire Bible. During a broadcast on John 3, he came to the famous verse 16 and asked the question, "What does it mean to believe?" Is it a state of mind? Is it a one time decision? Or is it something more?
He defined "to believe in Jesus" as something more that an intellectual decision. When I believe in Jesus, I trust Him in such a way that if He were to let me go, I would helplessly fall. Dr. McGee went on to liken this to a chair. If I were to stand next to a chair and say that I believe that the chair would hold me yet do not actually sit in the chair, I am not placing any trust in the chair. It is not until I sit in the chair that I am "believing in" or trusting the chair. Standing next to it, I can say whatever I would like--it means nothing until I place my trust in the chair.
It is the same way with belief in Jesus. If I stand on the outside of faith and say that I believe in Jesus then I am placing no more trust in Him than I was when I was referring to the chair. It is only when we rest in Jesus' arms and place our trust in Him that we are believing in Him. If He were to let us go, we would helplessly fall.
The reality is that there is a God who loves us and will not abandon us (Hebrews 13:5). We must move past mere intellectual thoughts about God and fully trust in Him.