John Gill Commentary John 8:51

"Verily, verily, I say unto you, If a man keep my word, he shall never see death." — John 8:51 (ASV)

Verily, verily, I say unto you
This is truth, and may be depended upon, as coming from the "Amen", and faithful witness:

if a man keep my saying ;
or doctrine, receives the Gospel in the love of it, obeys it from his heart, and cordially embraces and firmly believes it; and retains and holds it fast, having a spiritual and comfortable experience of the doctrines of Christ, and yielding a cheerful and ready obedience to his commands and ordinances, in faith and love:

he shall never see death ;
the second death, eternal death, which is an everlasting separation of a man, body and soul, from God: this death shall have no power on such a person, he shall never be hurt by it; and though he dies a corporeal death, that shall not be a curse, a penal evil to him; nor shall he always lie under the power of it, but shall rise again, and live in soul and body, for ever with the Lord: seeing and tasting death, as in (John 8:52) , are Hebraisms expressive of dying.