After Jesus spoke these words, there were people in the crowd that believed on Him. Therefore, Jesus addressed these new believers. He told them if they continued to be in His word, then they would truly be His disciples. When they know the truth, then they would be spiritually free.
There is a great difference between merely believing Jesus and believing in Him or on Him. The difference is found in the Greek text of this passage. Verse 30 speaks of those who really did trust Jesus and who had personally committed themselves to Him. John says, “Even as he spoke, many put their faith in him.” In the following verse, the phrase is changed so that we find ourselves reading of those who merely “believed him.” The Authorized Version and the Revised Standard Version translate the phrases identically (either “believed on” or “believed in”). The NEB, the NASB, the NIV, and others preserve the distinction. The NEB is explicit in its translation. It says, “As he said this, many put their faith in him. Turning to the Jews who had believed him, Jesus said…” The NIV says, “Many put their faith in Him. To the Jews who had believed him, Jesus said…” In other words, the verses distinguish between those listeners who had believed on Jesus unto salvation and those who had merely believed certain things that he had said.
When noting these contrasts between those who merely believe doctrines and those who believe on Jesus personally, this is pointed out elsewhere. In the sixth chapter we find Jesus calling for saving faith by saying, “The work of God is this: to believe in the one He has sent” (verse 29) Immediately after this, those who refuse to believe on that level ask Jesus “What miraculous sign will you give then, that we may see and believe?” (Verse 30) They believe, but they do not believe in.
People can believe on the intellectual level only. It is another thing to believe in the biblical sense, which brings about commitment to the Lord. It also brings about trusting Him to carry you through life’s challenges. When Jesus speaks to the weak in faith, He says, “If you hold to my teaching, you are my disciples. Then you will know the truth, and the truth will set you free.