The Fathers who believed in a young earth and a 6 literal day creation week are: Lactantius (250-325 AD) who viewed the creation days as 24-hour days.
Victorinus (304 AD) affirmed that the first day of creation was divided into 12 hours for day and 12 hours for night.
Ephrem the Syrian (306-373) one of the few fathers who knew Hebrew, also held the 12-hour view of Victorinus,
Basil of Caesarea (329-379) was specific that creation happened quickly, and in 24-hour days. Gregory of Nyssa in his own Hexaemeron upholds Basil's viewpoint as does Ambrose (338-397).
Allegorists such as Augustine, Clement of Alexandria, and Origen, while they did not not hold the literal 6-day view believed God created in an instant and that the earth was young by the secular standards of the day. Also, the long list of millennialists including Justin Martyr and Irenaeus all held a literal view due to their analogy with "a day is a thousand years and a thousand years a day". The view of the early Church was most definitely: we have a young earth and it was created either in an instant or in 6 days.
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