God’ (5:5), is now further and more fully described, particularly with regard to his mission on earth. This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. Various interpretations have been given to these phrases, which Plummer calls the ‘most perplexing’ in the letter. There can be little doubt that John was using phraseology which was already familiar to his readers, either through his own teaching or through that of the false teachers,
Pages 177–178