Albert Barnes Commentary 1 John 3:14

"We know that we have passed out of death into life, because we love the brethren. He that loveth not abideth in death." — 1 John 3:14 (ASV)

We know that we have passed from death unto life. From spiritual death (see the notes on Ephesians 2:1) to spiritual life; that is, that we are true Christians. Because we love our fellow believers. 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 on John 13:35.

But how easy it would seem to be to apply such a test of piety as this! Who cannot accurately judge his own feelings and determine whether he loves a Christian because he bears the name and image of the Saviour—and loves him more precisely 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, rather 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 this 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 (see the notes on 1 John 3:6).

As love for the Christian fellowship is essential to true piety, it follows that he who does not have that love remains unconverted, or is in a state of spiritual death. He is by nature dead in sin, and unless he has evidence that he has been brought out of that state, he remains or abides in it.