He that saith, I know Him - He who professes to be acquainted with the Saviour, or who professes to be a Christian.
And keepeth not his commandments - What he has appointed to be observed by his people; that is, he who does not obey him.
Is a liar - Makes a false profession; professes to have that which he really has not. Such a profession is a falsehood, because there can be no true religion where one does not obey the law of God.