In the book of John, we read that one of the main objections to Jesus' claims was that he was his own witness - the Pharisees told him in chapter 8 that "you are bearing witness about yourself, your witness is not true." And Jesus acknowledges that he's his own witness, but that doesn't mean he's not telling them the truth - in fact, he's the only one who can bear witness to the things he's telling them, because he's the only one who's been where he has been: in Heaven with the Father.

Jesus requires that we believe his testimony not because he conclusively proved everything he said, but that he proved he was who he said he was: the Christ, the son of God. He proved those things by his miracles, and by the righteous example he displayed every day of his life. Jesus' argument is that once he has proved who he is, then anyone who believes should be willing to accept whatever he says about the kingdom of Heaven, because who else would know these things but God?

Truly believing in Jesus doesn't allow us to cherry-pick what Jesus taught. We can't say "You are the son of God... but I think you're wrong about this." True faith calls us to put our trust in the words delivered by Jesus, the Christ.