John 8.31-32:
So Jesus said to the Jews who had believed him, “If you abide in my word, you are truly my disciples, (32) and you will know the truth, and the truth will set you free.” (ESV)
Whenever the New Testament describes someone believing in Jesus, it uses a variety of verb forms to do so. There are cases when the verb for believing in Jesus indicates a specific moment in time of believing, with completed action. Other times the verb forms indicate continuous, ongoing action of believing. The way this all works together is to conclude that faith in Jesus begins in a person’s life at a specific moment in time, and thereafter continues unending into the future. Such is the nature of genuine faith in Jesus.
Verse 31 above describes the people who believed in Jesus as believing in such a way that it began in the past and continues ongoing into the future. So what is believing in Jesus? As Jesus speaks of it in verse 24 earlier in this chapter, it is in effect agreeing with and accepting as true that Jesus is who he says he is. That he is the Son of God. In its fullest sense, it is trusting that Jesus is the Mediator between God and man, and that by believing and trusting in him as such Mediator and Savior, a person will receive the promise of eternal life with Jesus in heaven.