Are entered into the world] rather, 'are gone out into the world': cp. RV; i.e. they were formerly members of the Church, but have apostatised: cp. 1 John 2:19. To confess that Jesus Christ is come in the flesh is with St. John the central fact of Christian belief. Not to recognise in Jesus the authentic revelation of the infinite God and the highest ideal of humanity, can, in his opinion, imply only moral depravity.
Related Commentaries of 2 John 1:7
2 John 1:7
7 For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist.