Dates: c. 325 (the Chronicon) · Wikipedia
Summary: Where Justin charged the rabbis with cutting messianic
proof-texts, Eusebius brings the charge to the chronology. In Book 1 of the Chronicon he sets
the Septuagint, Hebrew, and Samaritan reckonings of Genesis 5–11 side by side, finds the Greek and the Hebrew
agreeing in the total lifespans but disagreeing — by exactly a hundred years apiece — in the begetting-ages of
the antediluvian patriarchs, and concludes that the Septuagint preserves the original. The shorter Hebrew
begetting-ages, he suspects, are a deliberate Jewish abridgement: "they made bold to shorten the time before the
fathering of children to encourage early marriages."
"The Greek translation [of the Bible] also differs from the Hebrew, though not so much from the Samaritan [version]. There is disagreement [in chronology in the versions] up to the flood, but thereafter, until the time of Abraham, the versions are in harmony. The text we use was translated collectively by seventy Hebrew men from their language into Greek during the reign of Ptolemaeus Philadelphus. [Their translation] was placed in the library in the city of Alexandria, where it was carefully preserved."
— Eusebius, Chronicon, Book 1 (§§24–25), trans. from the Armenian, source. Eusebius treats the divergence as a problem to be solved by comparison rather than asserted: he tabulates each patriarch from Adam to Noah in all three recensions. The Septuagint reckons the period from Adam to the death of Noah at "2,242 years"; the Hebrew of the Jews at "1656 years"; the Samaritan at "1,307 years."
"There is a 586 year discrepancy between this version and the Septuagint. The difference is in the number of years each man from Adam to Noah lived before fathering children. [The versions agree] only for the times of Jared, Methuselah, and Lamech. This circumstance suggests to us that the text which we use [i.e. the Septuagint] is the best. From the longer period assigned to Jared and his descendants [in the Hebrew version] it is clear that the periods of their predecessors, similarly, should be the same as in the Septuagint version. By adding one hundred years the discrepancy observed between the later and more recent generations in the Hebrew and the Septuagint versions is eliminated."
— Eusebius, Chronicon, Book 1 (§25), trans. from the Armenian, source. His argument turns on an internal consistency check. For each patriarch the begetting-age plus the years-lived-after sum to the same total lifespan in both the Hebrew and the Greek; only the begetting-age differs, and there the Hebrew runs short by precisely a century in the disputed generations — while at Jared, Methuselah, and Lamech, where Hebrew and Greek happen to coincide, no abridgement appears. Since those three set the pattern, Eusebius infers the other Hebrew figures have been lowered from an original that matched the Septuagint.
"It is only the number of years before their sons were born which is shorter in the Jewish copies. Therefore we suspect that this was something which the Jews did. They made bold to shorten the time before the fathering of children to encourage early marriages. For if these ancestors lived such long lives, marrying early and fathering children early as their version clearly states, who would not want to emulate them by marrying early?"
— Eusebius, Chronicon, Book 1 (§25), trans. from the Armenian, source. The motive Eusebius assigns is moral-exemplary, not anti-Christian: the rabbis are imagined lowering the begetting-ages so the long-lived patriarchs would model youthful marriage. This is a milder charge than the Messiah-timing motive later writers (Julian of Toledo, Agapius) would attach to the same chronological gap — but it is the patristic root of the claim that the Hebrew numbers, not the Greek, are the altered ones.