Interactive Online Study Notes

Now by this we know that we know Him, if we keep His commandments.

As one commentator explains in the language form: "if we keep on keeping His commandments."

This is such a simple statement but so clear. I have seen over the years countless ones who appear to talk the talk, and come along to meetings etc yet they "fall" into sin and compromise regularly. But this verse is quite clear, they do not know Him.
Know =

1. to learn to know, come to know, get a knowledge of perceive, feel
   1. to become known
2. to know, understand, perceive, have knowledge of
   1. to understand
   2. to know
3. Jewish idiom for sexual intercourse between a man and a woman
4. to become acquainted with, to know 

This "knowing" Him is more than head knowledge, it is meaning an intimate relationship with Him as we can see in the Greek root of the word: it is a Jewish idiom for sexual intercourse between a man and a woman. Recently at church we've been looking into the Hebrew vs Greek mindset and the Hebrew "knowing" is quite different, here's a quote from the sermon:
Knowledge to the Hebraic mind was not just the accumulation of data, facts, information, and ideas, as the Western thought belief system declares. It is about meeting and interacting with the living God and obeying Him.  
Therefore they do not know Him in the Hebraic sense of the term.