Augustine of Hippo : Here John testifies that no one has life unless he has Christ. — AGAINST JULIAN 6.9.27 Bede : He who has the Son, has life, etc. So that it may not seem too little to say that life is in the Son, he added that the Son Himself is life. Which the Son, glorifying the Father, also showed when He said: For as the Father has life in Himself, so He has granted the Son to have life in Himself (John V). But how the same life which is common to both the Son and the Father also enlightens believers, the same Son elsewhere intimates in prayer to the Father: As You have given Him authority over all flesh, that He should give eternal life to as many as You have given Him. This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent (John XVII). — Commentary on the Catholic Epistles Oecumenius : He who has the Son has life; he who does not have the Son of God does not have life. Therefore, whoever has the Son through holy baptism has life. For as many as we have been baptized into Christ, we have also, according to His command, put on Christ. But whoever does not have the Son of God through baptism does not have life, but is dead. For indeed, receiving the dead through sins, He awakens them through holy baptism. (Rom. 6:4) How? As it has often been said, that having been buried with Christ through the immersion that takes place in baptism, he is dead to the world (Gal. 6:14; Col. 2:20), or to worldly desires, and now lives not for himself, but for Christ (Gal. 2:20), walking according to His commandments in newness of life (Rom. 6:4), and no longer giving sin an opportunity to enter covertly. — Commentary on 1 John Tertullian : We believe not the testimony of God in which He testifies to us of His Son. "He that hath not the Son, hath not life." And that man has not the Son, who believes Him to be any other than the Son. — Against Praxeas