In lecture 13 of DIM, Peikoff discusses history. As one might expect, Peikoff doesn't like Christianity.
1. He claims that Constantine made Christianity the religion of the Roman Empire in 315. Apparently, Peikoff is referring to the Edict of Milan (which was in 313). According to most historians, this edict fell short of making Christianity the official religion, which didn't happen until Emperor Theodosius (347-395).
2. He claims that Christians attempted to obliterate pagan culture. Actually, the Christians tended to admire pagan culture and one of the points of the Protestant reformation was that the early Christians had embraced too much pagan thought. For example, here is Lactantius (c. 240-320).
"Lactantius, in a spirit not unlike that of Constantine himself, cites indiscriminately texts from Holy Writ and from pagan seers, poets, and philosopher; utterances of Orpheus, the Sibyl, Hermes Trismegistus, Vergil and Ovid, Thales, Aristotle and Cicero being quoted alongside pasages from the Hebrew Scriptures in support of the belief." [C. N. Cochrane, Christianity and Classical Culture, p. 211.]
As another example, the greatest early Christian poet Prudentius (348- c. 405) imitated the pagan hymns of Horace.