Albert Barnes on 1 John 2:3

And hereby we do know that we know him. To wit, by that Which follows, we have evidence that we are truly acquainted with him, and with the requirements of his religion; that is, that we are truly his friends. The word him, in this verse, seems to refer to the Saviour. On the meaning of the word know, Jn 17:3. The apostle had stated in the previous part of this epistle some of the leading points revealed by the Christian religion, and he here enters on the consideration of the nature of the evidence required to show that we are personally interested in it, or that we are true Christians. A large part of the epistle is occupied with this subject. The first, the grand evidence--that without which all others would be vain--he says is, that we keep his commandments.

If we keep his commandments. Jn 14:15. Comp. Jn 14:23,24, 15:10,14.