A.T. Robertson Commentary John 6:53

"Jesus therefore said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man and drink his blood, ye have not life in yourselves." — John 6:53 (ASV)

Except ye eat (εαν μη φαγητε). Negative condition of third class with second aorist active subjunctive of εσθιω. Jesus repeats the statement in verses 50,51. Note change of μου (my) in verse 51 to του υιου του ανθρωπου with same idea.

And drink his blood (κα πιητε αυτου το αιμα). Same condition with second aorist active subjunctive of πινω. This addition makes the demand of Jesus seem to these Jews more impossible than before if taken in a baldly literal sense. The only possible meaning is the spiritual appropriation of Jesus Christ by faith (verse 47), for "ye have not life in yourselves" (ουκ εχετε ζωην εν εαυτοις). Life is found only in Christ.