15 “He that believes in me, though he were dead, yet shall he live: and whosoever lives and believes in me shall never die.” What means this? “He that believes in me, though he were dead,” just as Lazarus is dead, “yet shall he live;” for He is not the God of the dead, but of the living. Such was the answer He gave the Jews concerning their fathers, long ago dead, that is, concerning Abraham, and Isaac, and Jacob: I am the God of Abraham, and the God of Isaac, and the God of Jacob: He is not the God of the dead, but of the living; for all live unto Him. Believe then, and though thou were dead, yet shall you live: but if you believe not, even while you live you are dead.
Let us prove this likewise, that if you believe not, though you live you are dead. To one who was delaying to follow Him, and saying, “Let me first go and bury my father,” the Lord said, “Let the dead bury their dead; but come thou and follow me.” There was there a dead man requiring to be buried, there were there also dead men to bury the dead: the one was dead in the flesh, the others in soul. And how comes death on the soul? When faith is wanting. How comes death on the body?
When the soul is wanting. Therefore your soul's soul is faith. “He that believes in me,” says Christ, though he were dead in the flesh, yet shall he live in the spirit; till the flesh also rise again, never more to die. This is “he that believes in me,” though he die, “yet shall he live. And whosoever lives” in the flesh, “and believes in me,” though he shall die in time on account of the death of the flesh, “shall never die,” because of the life of the spirit, and the immortality of the resurrection.
Such is the meaning of the words, “And whosoever lives and believes in me shall never die. Believest thou this? She says unto Him, Yea, Lord, I have believed that You are the Christ, the Son of God, who hast come into the world.” When I believed this, I believed that You are the resurrection, that You are the life: I believed that he that believes in You, though he die, yet shall he live; and whosoever lives and believes in You, shall never die.
Source: Tractates on the Gospel of John (New Advent)