Aha! A Coulter factual error.
Constantine was the first Roman Emperor to endorse Christianity. He legalized and strongly supported Christianity with the Edict of Milan (313 AD), but he neither made paganism illegal nor made Christianity the state religion. Theodosius (379-395) declared Christianity the state religion of Rome; he made all pagan religions illegal. In 410, the Visigoths, captured and sacked Rome. In 455, the Vandals, another Germanic tribe, conquered Rome. Finally, in 476, Odoacer deposed the Roman emperor and made himself emperor, which essentially ends the Roman Empire.
Thus Christianity did not "topple" the Roman Empire, it was there at its fall at the hands of pagan barbarians.
Now, were we liberals, we would state that Coulter's piece was "ridden with factual errors", and thus nothing she writes is valid.
The prophet Daniel disagrees with you, as does a correct reading of Revelation.