We know that we have passed from death unto life - From spiritual death (Notes, Ephesians 2:1) to spiritual life; that is, that we are true Christians.
Because we love the brethren - The sentiment here is, that it is an infallible evidence of true piety if we love the followers of Christ as such. See this sentiment illustrated in the notes at John 13:35. But how easy it would seem to be to apply such a test of piety as this! Who cannot judge accurately of his own feelings, and determine whether he loves a Christian because he bears the name and image of the Saviour - loves him the more just in proportion as he bears that image? Who cannot, if he chooses, look beyond the narrow bounds of his own sect, and determine whether he is pleased with the true Christian character wherever it may be found, and whether he would prefer to find his friends among those who bear the name and the image of the Son of God, than among the people of the world? The Saviour meant that his followers should be known by this badge of discipleship all over the world, John 13:34-35. John says, in carrying out the sentiment, that Christians, by this test, may know “among themselves” whether they have any true religion.
He that loveth not his brother abideth in death - He remains dead in sins; that is, he has never been converted. Compare the notes at 1 John 3:6. As love to the Christian brotherhood is essential to true piety, it follows that he who has not that remains unconverted, or is in a state of spiritual death. He is by nature dead in sin, and unless he has evidence that he is brought out of that state, he “remains” or “abides” in it.