Hey, great question! My theology professor once wrote:
"Our knowledge of someone is precisely the DIFFERENCE that person has made within us as a result of our meeting him/her. To know God is to be profoundly ALTERED by God through having encountered him."
The more you know a person, the more effect there is on your life. That's the principle. When you truly know a person, your life is made different forever. I'll give you an example from my own life. I've dated my wife for 13 years (we've been married for 5). I can confidently say that since I've gotten to know her, she's made the biggest impact in my life. I've been made different forever. If my life wasn't much different after 13 years, then I really couldn't say I know my wife. I may know facts about her (e.g. her favourite colour is blue; she likes sushi; she is the eldest child, etc.), but I don't really know her intimately.
With respect to our relationship with God, the same principle holds true. We know God when we have been made different forever through our encounter with Him. Sadly, many people know FACTS about God (e.g. believe that Jesus did this and that), but they don't know God PERSONALLY - God has not had a profound, everlasting impact in their life. When you know God, the direction of your life MUST change because that's the kind of IMPACT God makes on a person. He works in a person's heart, so she no longer desire to sin, but desire to live in holiness and obedience to Him. When you truly know God, your priorities; life direction; behaviour; words; innermost desires change as a result of your relationship with Him.
That's what I mean.
Thanks for asking. Keep 'em coming.
[Answered by Pastor HM]