Knowing means having that fruitful acquaintance. Imparting knowledge means helping another gain that intimacy.
So, when the preacher preaches the gospel, calling us to repentance, he or she doesn't just repeat the facts about the need for repentance or the facts about Jesus Christ's good news. The good preacher finds a way to make intimacy with the matter of the sermon possible in a way that will bear fruit in our lives. That can't be done without the facts, but the facts aren't enough to constitute knowledge of the gospel.
The good preacher knows the audience, knows the text or texts to be used in the sermon, and knows how best to make those texts preach the message of repentance and salvation. He or she doesn't recite the fact about our need for new life, but uses the sermon to urge us to new life, to invite us—to call or even, one might say, to tempt, us. The good preacher uses eros, insatiable productive desire, to bring us to the gospel.
Having come to the gospel, however haltingly or weakly, in whatever injured and incomplete way we have come, when we speak of our new life and its fruits, it is completely appropriate to use the biblical language of knowledge: "I know . . ."
Saying merely "I believe" could betray the relationship of intimacy that constitutes the knowledge of divine things by turning that relationship into merely a stale set of facts. Only "I know" is likely to remain true to our relationship with what has been revealed to and in us.