I understand your question about John 8:30-31, but it seems to me that we are left with a certain amount of doubt here.
We know from James 2:19 that there is a kind of intellectual belief that is not saving faith. We know from the parable of the 4 soils that some receive the seed, but fail to produce fruit (Mark 4). In John’s Gospel we have several problematic statements about those who “believe” or “follow” our Lord, but do not persist. John 2:23-25 is the first instance, followed by John 6:60-66. In John 8, Jesus is speaking to “the Jews who believed on Him” (8:31), but our Lord seems to instruct and warn them that they will really show whether or not they are His followers by abiding in His Word (8:31-32). You should remember that prior to our Lord’s crucifixion and resurrection they were still looking ahead to the cross, and not back to His death and resurrection as a historical fact.
By the way, we should not forget Matthew 7:15-27. There were those false prophets who claimed not only to know our Lord, but to perform miracles in His name, but Jesus said He did not know them. They did not bear the fruit of a true believer.
Therefore, a mere intellection belief about Jesus is not enough to save one, nor is a mere profession that you do know Him. There must be a personal commitment to trust in His shed blood, and the fruit should follow.
I would also be quick to add that it is our Lord alone who saves and who keeps us, which is very clear in John (John 5:21; 6:28-29, 65; 10:26-27). We do not believe in Him and strive to follow Him in order to be saved; but those who truly believe in Him will follow Him (John 10:27).
Jesus did not simply call men to believe, be saved, and go their merry way; He called men to trust in Him, and to become His disciples, and to carry out His work on this earth.